Document "git-reset --merge"

Document "git-reset --merge"

The commit log message for the feature made it sound as if this is a saner
version of --mixed, but the use case presented makes it clear that it is a
better variant of --hard when your changes and somebody else's changes are
mixed together.

Perhaps we would want to rewrite the example that shows the use of --hard
not to talk about recovering from a failed merge?

Signed-off-by: Junio C Hamano <gitster@pobox.com>