mailmap: update manpage link

mailmap: update manpage link

Patch series "Update .mailmap for my work address and fix manpage".

While updating mailmap for the going-away address, I also found that on
current systems the manpage linked from the header comment changed.

And in fact it looks like the git mailmap feature got its own manpage.

This patch (of 2):

On recent systems the git-shortlog manpage only tells people to
    See gitmailmap(5)

So instead of sending people on a scavenger hunt, put that info into the
header directly.  Though keep the old reference around for older systems.

Link: https://lkml.kernel.org/r/20230704163919.1136784-1-heiko@sntech.de
Link: https://lkml.kernel.org/r/20230704163919.1136784-2-heiko@sntech.de
Signed-off-by: Heiko Stuebner <heiko.stuebner@vrull.eu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>