doc: describe mergetool configuration in git-mergetool(1)

In particular, this describes mergetool.hideResolved, which can help
users discover this setting (either because it may be useful to them
or in order to understand mergetool's behavior if they have forgotten
setting it in the past).

Tested by running

	make -C Documentation git-mergetool.1
	man Documentation/git-mergetool.1

and reading through the page.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-mergetool.txt

@@ -99,6 +99,10 @@ success of the resolution after the custom tool has exited.
 	(see linkgit:git-config[1]).  To cancel `diff.orderFile`,
 	use `-O/dev/null`.
 
+CONFIGURATION
+-------------
+include::config/mergetool.txt[]
+
 TEMPORARY FILES
 ---------------
 `git mergetool` creates `*.orig` backup files while resolving merges.