Document limited recursion pathspec matching with wildcards

It's actually unlimited recursion if wildcards are active regardless
--max-depth

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-grep.txt

@@ -79,6 +79,9 @@ OPTIONS
 --max-depth <depth>::
 	For each <pathspec> given on command line, descend at most <depth>
 	levels of directories. A negative value means no limit.
+	This option is ignored if <pathspec> contains active wildcards.
+	In other words if "a*" matches a directory named "a*",
+	"*" is matched literally so --max-depth is still effective.
 
 -w::
 --word-regexp::

tree-walk.c

@@ -661,6 +661,9 @@ match_wildcards:
 		/*
 		 * Match all directories. We'll try to match files
 		 * later on.
+		 * max_depth is ignored but we may consider support it
+		 * in future, see
+		 * http://thread.gmane.org/gmane.comp.version-control.git/163757/focus=163840
 		 */
 		if (ps->recursive && S_ISDIR(entry->mode))
 			return entry_interesting;