[Feature request]: mention if HOMEBREW_EMACS_PLUS_31_REVISION is set

[tzz]

Feature request description

I had forgotten this set in my shell for months

% echo $HOMEBREW_EMACS_PLUS_31_REVISION
81c21d89ede8dfa664b7a3700acd7bf4c9fa54aa

and every build was pinned to the same revision of course:

==> Fetching d12frosted/emacs-plus/emacs-plus@31
==> Cloning https://github.com/emacs-mirror/emacs.git
Updating /Users/tzz/Library/Caches/Homebrew/emacs-plus@31--git
'origin/HEAD' is unchanged and points to 'master'
refs/remotes/origin/master
==> Checking out revision 81c21d89ede8dfa664b7a3700acd7bf4c9fa54aa
HEAD is now at 81c21d89ede Ensure .dir-locals-2.el behavior as documented (bug#75890)
HEAD is now at 81c21d89ede Ensure .dir-locals-2.el behavior as documented (bug#75890)

There's nothing in the output to indicate HOMEBREW_EMACS_PLUS_31_REVISION is set. Could we add something for forgetful people like me?

Thank you!