Redo examples to be less confusing

Author: jyn514

src/building/bootstrapping.md

@@ -14,7 +14,7 @@ It must have been written in a different language. In Rust's case it was
 only way to build a modern version of rustc is a slightly less modern
 version.
 
-This is exactly how `x.py` works: it downloads the current `beta` release of
+This is exactly how `x.py` works: it downloads the current beta release of
 rustc, then uses it to compile the new compiler.
 
 ## Stages of bootstrapping
@@ -134,10 +134,16 @@ The convention `x.py` uses is that:
 - The "stage (N+1) compiler" is assembled from "stage N artifacts". This
   process is called _uplifting_.
 
-For example, `x.py build --stage 0` means to build with the beta `rustc`, and
-`test --stage 0` isn't really meaningful (it's not running tests on your changes,
-but on `beta`, so it always fails). Similarly, `doc --stage 0` means to
-document using the beta `rustdoc`.
+#### Examples
+
+- `x.py build --stage 0` means to build with the beta `rustc`.
+- `x.py doc --stage 0` means to document using the beta `rustdoc`.
+- `x.py test --stage 0 library/std` means to run tests on the standard library
+  without building `rustc` from source ('build with stage 0, then test the artifacts').
+- `x.py test --stage 0 src/test/ui` is not meaningful: it runs tests on the
+  _beta_ compiler and doesn't build `rustc` from source.
+
+### Building vs. Running
 
 Note that this implies the stage N compiler is _not_ the same as the compiler
 built by `build --stage N compiler/rustc` -- that's 'stage N artifacts'