maint: Tweak comments in mailmap regarding SPF

maint: Tweak comments in mailmap regarding SPF

Documenting that we should not add new lines to work around SPF
rewrites sounds foreboding; the intent is instead that new lines here
are okay, but indicate a second problem elsewhere in our build process
that we should also consider fixing at the same time, to keep the
section from growing without bounds.  While we have been doing that
for qemu-devel for a while, we jut recently fixed that for qemu-block:
https://git.linaro.org/people/pmaydell/misc-scripts.git/commit/?id=f9a317392

Mentioning DMARC alongside SPF may also help people grep for this
scenario, as well as documenting the 'git config' workaround that can
be used by submitters to avoid the munging issue in the first place.

Note the subtlety: 'git commit' sets authorship information based on
user.name and user.email (where name is usually unquoted); while 'git
send-email' includes a body 'From:' line only when sendemail.from is
present but differs from authorship information.  Hence the use of
quotes in sendemail.from (not a semantic change to email, but enough
of a difference to add the body 'From:').

Fixes: 3bd2608d ("maint: Add .mailmap entries for patches claiming list authorship")
CC: Andrey Drobyshev <andrey.drobyshev@virtuozzo.com>
Cc: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Eric Blake <eblake@redhat.com>
Message-ID: <20230927143815.3397386-7-eblake@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>