Add documentation for RUSTC_EMIT

Author: Zoxc

src/building/bootstrapping.md

@@ -365,6 +365,16 @@ recompiling all dependencies.
 `CARGOFLAGS` will pass arguments to cargo itself (e.g. `--timings`). `CARGOFLAGS_BOOTSTRAP` and
 `CARGOFLAGS_NOT_BOOTSTRAP` work analogously to `RUSTFLAGS_BOOTSTRAP`.
 
+`RUSTC_EMIT` will cause `bootstrap` to pass `--emit` arguments on to `rustc`.
+You can specify which `--emit` formats which are used with a comma separated list prefixed with
+a crate name and `=`, for example: `rustc_arena=llvm-ir,asm`.
+You can also substitute `*` for the crate name to apply it to all crates.
+You can combine multiple such specifiers by seperating them with semi-colons like
+`rustc_arena=llvm-ir,asm;rustc_span=asm;*=llvm-bc`.
+The emitted files will be stored inside the `emit` directory in the target specific build directory.
+`RUSTC_EMIT` will not cause `bootstrap` to rebuild any crates so you may need to modify the crates
+you're interested in.
+
 `--test-args` will pass arguments through to the test runner. For `tests/ui`, this is
 compiletest; for unit tests and doctests this is the `libtest` runner. Most test runner accept
 `--help`, which you can use to find out the options accepted by the runner.