Building a fully-functional compiler requires x build library

Author: Zalathar

src/doc/rustc-dev-guide/src/building/how-to-build-and-run.md

@@ -227,15 +227,18 @@ Once you've created a `bootstrap.toml`, you are now ready to run
 probably the best "go to" command for building a local compiler:
 
 ```console
-./x build rustc
+./x build library
 ```
 
-What this command does is build `rustc` using the stage0 compiler and stage0 `std`.
+What this command does is:
+- Build `rustc` using the stage0 compiler and stage0 `std`.
+- Build `library` (the standard libraries) with the stage1 compiler that was just built.
+- Assemble a working stage1 sysroot, containing the stage1 compiler and stage1 standard libraries.
 
 To build `rustc` with the in-tree `std`, use this command instead:
 
 ```console
-./x build rustc --stage 2
+./x build library --stage 2
 ```
 
 This final product (stage1 compiler + libs built using that compiler)