Fix odd markup in --diff-filter documentation

Instead of using the regex-like bracket expression, use grouping to make
it more consistent with other similar places. The brackets now have the
same meaning as in other documentation (i.e., the argument is optional).

Signed-off-by: Štěpán Němec <stepnem@gmail.com>
Mentored-and-Acked-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/diff-options.txt

@@ -248,13 +248,13 @@ endif::git-log[]
 	If `n` is specified, it has the same meaning as for `-M<n>`.
 
 ifndef::git-format-patch[]
---diff-filter=[ACDMRTUXB*]::
+--diff-filter=[(A|C|D|M|R|T|U|X|B)...[*]]::
 	Select only files that are Added (`A`), Copied (`C`),
 	Deleted (`D`), Modified (`M`), Renamed (`R`), have their
 	type (i.e. regular file, symlink, submodule, ...) changed (`T`),
 	are Unmerged (`U`), are
 	Unknown (`X`), or have had their pairing Broken (`B`).
-	Any combination of the filter characters may be used.
+	Any combination of the filter characters (including none) can be used.
 	When `*` (All-or-none) is added to the combination, all
 	paths are selected if there is any file that matches
 	other criteria in the comparison; if there is no file