Improve wording for section heading

also adds another example of what not to do

Author: jyn514

src/building/bootstrapping.md

@@ -146,14 +146,19 @@ The convention `x.py` uses is that:
   `compiletest` on it. If you're working on the compiler, this is normally the
   test command you want.
 
-#### Non-examples
+#### Examples of what *not* to do
 
 - `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. Use `test src/test/ui`
   instead, which builds stage 1 from source.
 - `x.py test --stage 0 compiler/rustc` builds the compiler but runs no tests:
   it's running `cargo test -p rustc`, but cargo doesn't understand Rust's
   tests. You shouldn't need to use this, use `test` instead (without arguments).
+- `x.py build --stage 0 compiler/rustc` builds the compiler, but does not make
+  it usable: the build artifacts are not uplifted ([#73519]). Use `build
+  library/std` instead which puts the compiler in `stage1/rustc`.
+
+[#73519]: https://github.com/rust-lang/rust/issues/73519
 
 ### Building vs. Running