Remove unnecessary detail in building chapter

Most of these details were not helpful or necessary for building the compiler for the first time.
This section comes very early in the guide and is meant to be a tutorial, so being concise is very important.

Author: jyn514

src/building/how-to-build-and-run.md

@@ -24,52 +24,24 @@ cd rust
 ## Create a `config.toml`
 
 To start, run `./x.py setup`. This will do some initialization and create a
-`config.toml` for you with reasonable defaults. These defaults are specified
-indirectly via the `profile` setting, which points to one of the TOML files in
-`src/bootstrap/defaults.`
+`config.toml` for you with reasonable defaults.
 
 Alternatively, you can write `config.toml` by hand. See `config.toml.example`
-for all the available settings and explanations of them. The following settings
-are of particular interest, and `config.toml.example` has full explanations.
-
-You may want to change some of the following settings (and possibly others, such as
-`llvm.ccache`):
-
-```toml
-[llvm]
-# Whether to use Rust CI built LLVM instead of locally building it.
-download-ci-llvm = true     # Download a pre-built LLVM?
-assertions = true           # LLVM assertions on?
-ccache = "/path/to/ccache"  # Use ccache when building LLVM?
-
-[rust]
-debug-logging = true        # Leave debug! and trace! calls in rustc?
-incremental = true          # Build rustc with incremental compilation?
-```
-
-If you set `download-ci-llvm = true`, in some circumstances, such as when
-updating the version of LLVM used by `rustc`, you may want to temporarily
-disable this feature. See the ["Updating LLVM" section] for more.
-
-["Updating LLVM" section]: ../backend/updating-llvm.md#feature-updates
+for all the available settings and explanations of them. See `src/bootstrap/defaults` for common settings to change.
 
 If you have already built `rustc` and you change settings related to LLVM, then you may have to
 execute `rm -rf build` for subsequent configuration changes to take effect. Note that `./x.py
 clean` will not cause a rebuild of LLVM.
 
 ## What is `x.py`?
 
-`x.py` is the script used to orchestrate the tooling in the `rustc` repository.
-It is the script that can build docs, run tests, and compile `rustc`.
-It is the now preferred way to build `rustc` and it replaces the old makefiles
-from before. Below are the different ways to utilize `x.py` in order to
-effectively deal with the repo for various common tasks.
+`x.py` is the build tool for the `rust` repository. It can build docs, run tests, and compile the
+compiler and standard library.
 
 This chapter focuses on the basics to be productive, but
-if you want to learn more about `x.py`, read its README.md
-[here](https://github.com/rust-lang/rust/blob/master/src/bootstrap/README.md).
-To read more about the bootstrap process and why `x.py` is necessary,
-[read this chapter][bootstrap].
+if you want to learn more about `x.py`, [read this chapter][bootstrap].
+
+[bootstrap]: ./bootstrapping.md
 
 ### Running `x.py` slightly more conveniently
 
@@ -79,48 +51,14 @@ of a checkout. It also looks up the appropriate version of `python` to use.
 
 You can install it with `cargo install --path src/tools/x`.
 
-[bootstrap]: ./bootstrapping.md
-
 ## Building the Compiler
 
-To build a compiler, run `./x.py build`. This will build up to the stage1 compiler,
-including `rustdoc`, producing a usable compiler toolchain from the source
-code you have checked out.
-
 Note that building will require a relatively large amount of storage space.
 You may want to have upwards of 10 or 15 gigabytes available to build the compiler.
 
-There are many flags you can pass to the build command of `x.py` that can be
-beneficial to cutting down compile times or fitting other things you might
-need to change. They are:
-
-```txt
-Options:
-    -v, --verbose       use verbose output (-vv for very verbose)
-    -i, --incremental   use incremental compilation
-        --config FILE   TOML configuration file for build
-        --build BUILD   build target of the stage0 compiler
-        --host HOST     host targets to build
-        --target TARGET target targets to build
-        --on-fail CMD   command to run on failure
-        --stage N       stage to build
-        --keep-stage N  stage to keep without recompiling
-        --src DIR       path to the root of the Rust checkout
-    -j, --jobs JOBS     number of jobs to run in parallel
-    -h, --help          print this help message
-```
-
-For hacking, often building the stage 1 compiler is enough, which saves a lot
-of time. But for final testing and release, the stage 2 compiler is used.
-
-`./x.py check` is really fast to build the Rust compiler.
-It is, in particular, very useful when you're doing some kind of
-"type-based refactoring", like renaming a method, or changing the
-signature of some function.
-
 Once you've created a `config.toml`, you are now ready to run
 `x.py`. There are a lot of options here, but let's start with what is
-probably the best "go to" command for building a local rust:
+probably the best "go to" command for building a local compiler:
 
 ```bash
 ./x.py build library
@@ -144,6 +82,10 @@ see [the section on avoiding rebuilds for std][keep-stage].
 
 [keep-stage]: ./suggested.md#faster-builds-with---keep-stage
 
+Sometimes you don't need a full build. When doing some kind of
+"type-based refactoring", like renaming a method, or changing the
+signature of some function, you can use `./x.py check` instead for a much faster build.
+
 Note that this whole command just gives you a subset of the full `rustc`
 build. The **full** `rustc` build (what you get with `./x.py build
 --stage 2 compiler/rustc`) has quite a few more steps:
@@ -165,6 +107,8 @@ Instead, you can just build using the bootstrap compiler.
 ./x.py build --stage 0 library
 ```
 
+If you choose the `library` profile when running `x.py setup`, you can omit `--stage 0` (it's the default).
+
 ## Creating a rustup toolchain
 
 Once you have successfully built `rustc`, you will have created a bunch
@@ -273,7 +217,7 @@ in other sections:
   - `./x.py build` – builds everything using the stage 1 compiler,
     not just up to `std`
   - `./x.py build --stage 2` – builds everything with the stage 2 compiler including
-    `rustdoc` (which doesn't take too long)
+    `rustdoc`
 - Running tests (see the [section on running tests](../tests/running.html) for
   more details):
   - `./x.py test library/std` – runs the unit tests and integration tests from `std`