documentation: mention --no-use-mailmap and log.mailmap false setting

documentation: mention --no-use-mailmap and log.mailmap false setting

The log.mailmap setting may be explicitly set to false, which disables
the mailmap feature implicity. In practice, doing so is equivalent to
always using the previously undocumented --no-use-mailmap option on the
command line.

Accordingly, we document both the existence of --no-use-mailmap as
well as briefly discuss the equivalence of it to log.mailmap=False.

Signed-off-by: Ariadne Conill <ariadne@dereferenced.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>