doc: add description about pattern matching against directories

Before this patch, there is no explicit description about pattern
matching against directories, even though users may understand it from
"plain examples" in "hg help patterns".

This patch adds description about pattern matching against
directories.

mercurial/help/patterns.txt

@@ -29,6 +29,11 The latter expects null delimited patter
 feeds. Each string read from the file is itself treated as a file
 pattern.
 
+All patterns, except for ``glob:`` specified in command line (not for
+``-I`` or ``-X`` options) or ``.hgignore`` file, can match also
+against directories: files under matched directories are treated as
+matched.
+
 Plain examples::
 
   path:foo/bar   a name bar in a directory named foo in the root