- Suppress (some more) warnings when doing `brew install --quiet`
- Clarify `man brew` output that we don't suppress all warnings
for all commands with `--quiet`
While I was doing this I noticed references to the (soon to be
deprecated) `brew switch` so:
- remove these references in `install` output
- remove a reference in the documentation
- add a comment to remind me to deprecate `brew diy`, too
Fixes#9179
