Separate examples of what to do from examples of what not to do

Author: jyn514

src/building/bootstrapping.md

@@ -139,10 +139,18 @@ The convention `x.py` uses is that:
 - `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').
+    without building `rustc` from source ('build with stage 0, then test the
+  artifacts'). If you're working on the standard library, this is normally the
+  test command you want.
+- `x.py test src/test/ui` means to build the stage 1 compiler and run
+  `compiletest` on it. If you're working on the compiler, this is normally the
+  test command you want.
+
+#### Non-examples
+
 - `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 from source.
+  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).