mailmap: Update comments for with format and more detalis

Without having first read the git-shortlog man-page, the format
of .mailmap may not be immediately obvious. Add comments with pointers
to the man-page, along with other details.

Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/202008080013.58EBD83@keescook
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

.mailmap

@@ -2,11 +2,16 @@
 # This list is used by git-shortlog to fix a few botched name translations
 # in the git archive, either because the author's full name was messed up
 # and/or not always written the same way, making contributions from the
-# same person appearing not to be so or badly displayed.
+# same person appearing not to be so or badly displayed. Also allows for
+# old email addresses to map to new email addresses.
 #
+# For format details, see "MAPPING AUTHORS" in "man git-shortlog".
+#
+# Please keep this list dictionary sorted.
+#
+# This comment is parsed by git-shortlog:
 # repo-abbrev: /pub/scm/linux/kernel/git/
 #
-
 Aaron Durbin <adurbin@google.com>
 Adam Oldham <oldhamca@gmail.com>
 Adam Radford <aradford@gmail.com>