doc/go1.21: document clear builtin and init order changes
Also move all the language changes to the same part of the release notes.
For #56351
For #57411
Change-Id: Id1c51b5eb8f7d85e61a2ae44ee7d73bb13036631
Reviewed-on: https://go-review.googlesource.com/c/go/+/498755
TryBot-Bypass: Ian Lance Taylor <iant@google.com> Reviewed-by: Ian Lance Taylor <iant@google.com> Reviewed-by: Keith Randall <khr@golang.org>
Auto-Submit: Ian Lance Taylor <iant@google.com> Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Ian Lance Taylor <iant@golang.org>