bug#30610: clarify linux-mode usage from Lisp

[Noam Postavsky]

John Shahid <jvshahid@gmail.com> writes:

> Worth noting that this is my first patch to Emacs. Let me know if
> there's anything wrong with the way I submitted the patch.

The patch looks fine, but the commit message should have a ChangeLog
style entry as described in CONTRIBUTE under "Commit messages".  And if
you haven't assigned copyright for Emacs, it should also have a
'Copyright-paperwork-exempt: yes' line.  Note that we only accept up to
15 lines worth of unassigned patches.  If you want to start the
assignment process, fill out the form at [1] and send it to
fsf-records@gnu.org (they will reply with further instructions).

[1]: 
https://git.savannah.gnu.org/cgit/gnulib.git/tree/doc/Copyright/request-assign.program