config.txt: add an entry for log.showSignature

The configuration variable log.showSignature is mentioned in git-log's
manpage.  Document it in git-config's manpage as well.
Signed-off-by: NKyle Meyer <kyle@kyleam.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -2057,6 +2057,10 @@ log.showRoot::
 	Tools like linkgit:git-log[1] or linkgit:git-whatchanged[1], which
 	normally hide the root commit will now show it. True by default.
 
+log.showSignature::
+	If true, makes linkgit:git-log[1], linkgit:git-show[1], and
+	linkgit:git-whatchanged[1] assume `--show-signature`.
+
 log.mailmap::
 	If true, makes linkgit:git-log[1], linkgit:git-show[1], and
 	linkgit:git-whatchanged[1] assume `--use-mailmap`.