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>

Documentation/config/log.txt

@@ -40,4 +40,5 @@ log.showSignature::
 
 log.mailmap::
 	If true, makes linkgit:git-log[1], linkgit:git-show[1], and
-	linkgit:git-whatchanged[1] assume `--use-mailmap`.
+	linkgit:git-whatchanged[1] assume `--use-mailmap`, otherwise
+	assume `--no-use-mailmap`. False by default.

Documentation/git-log.txt

@@ -49,7 +49,7 @@ OPTIONS
 	Print out the ref name given on the command line by which each
 	commit was reached.
 
---use-mailmap::
+--[no-]use-mailmap::
 	Use mailmap file to map author and committer names and email
 	addresses to canonical real names and email addresses. See
 	linkgit:git-shortlog[1].