Clarify `raw_ptr<T>` advice for return values and unions.

1. It is fine to use regular C++ pointers for return values.
2. Regular C++ pointers should be preferred in C++ unions; however,
   update the advice to prefer `absl::variant` + `raw_ptr<T>` as a much
   safer alternative where possible.

Change-Id: I06673fcfb7a9a613c94b353bbfa5d51f328d17cd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4652409
Reviewed-by: Bartek Nowierski <bartekn@chromium.org>
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1165076}