Auto merge of rust-lang#77631 - jyn514:helpful-changelog, r=RalfJung

Make src/bootstrap/CHANGELOG.md more helpful

Addresses rust-lang@fe6fc55#r42949241.

r? `@RalfJung`
cc `@Mark-Simulacrum`

Author: bors

src/bootstrap/CHANGELOG.md

@@ -19,13 +19,13 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 
 - Add `x.py setup` [#76631](https://github.com/rust-lang/rust/pull/76631)
 - Add a changelog for x.py [#76626](https://github.com/rust-lang/rust/pull/76626)
-- Optionally, download LLVM from CI on Linux and NixOS
+- Optionally, download LLVM from CI on Linux and NixOS. This can be enabled with `download-ci-llvm = true` under `[llvm]`.
   + [#76439](https://github.com/rust-lang/rust/pull/76349)
   + [#76667](https://github.com/rust-lang/rust/pull/76667)
   + [#76708](https://github.com/rust-lang/rust/pull/76708)
 - Distribute rustc sources as part of `rustc-dev` [#76856](https://github.com/rust-lang/rust/pull/76856)
-- Make the default stage for x.py configurable [#76625](https://github.com/rust-lang/rust/pull/76625)
-- Add a dedicated debug-logging option [#76588](https://github.com/rust-lang/rust/pull/76588)
+- Make the default stage for x.py configurable [#76625](https://github.com/rust-lang/rust/pull/76625). This can be enabled with `build-stage = N`, `doc-stage = N`, etc.
+- Add a dedicated debug-logging option [#76588](https://github.com/rust-lang/rust/pull/76588). Previously, `debug-logging` could only be set with `debug-assertions`, slowing down the compiler more than necessary.
 - Add sample defaults for x.py [#76628](https://github.com/rust-lang/rust/pull/76628)
 - Add `--keep-stage-std`, which behaves like `keep-stage` but allows the stage
   0 compiler artifacts (i.e., stage1/bin/rustc) to be rebuilt if changed