mergetool,difftool: Document --tool-help consistently

Add an entry for --tool-help to the mergetool documentation.

Move --tool-help in the difftool documentation so that it is
listed immediately after --tool so that it is easier to find.

Signed-off-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-difftool.txt

@@ -66,6 +66,9 @@ of the diff post-image.  `$MERGED` is the name of the file which is
 being compared. `$BASE` is provided for compatibility
 with custom merge tool commands and has the same value as `$MERGED`.
 
+--tool-help::
+	Print a list of diff tools that may be used with `--tool`.
+
 --symlinks::
 --no-symlinks::
 	'git difftool''s default behavior is create symlinks to the
@@ -74,9 +77,6 @@ with custom merge tool commands and has the same value as `$MERGED`.
 	Specifying `--no-symlinks` instructs 'git difftool' to create
 	copies instead.  `--no-symlinks` is the default on Windows.
 
---tool-help::
-	Print a list of diff tools that may be used with `--tool`.
-
 -x <command>::
 --extcmd=<command>::
 	Specify a custom command for viewing diffs.

Documentation/git-mergetool.txt

@@ -64,6 +64,9 @@ variable `mergetool.<tool>.trustExitCode` can be set to `true`.
 Otherwise, 'git mergetool' will prompt the user to indicate the
 success of the resolution after the custom tool has exited.
 
+--tool-help::
+	Print a list of merge tools that may be used with `--tool`.
+
 -y::
 --no-prompt::
 	Don't prompt before each invocation of the merge resolution