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

@@ -1,57 +1,62
 Mercurial accepts several notations for identifying one or more files
 at a time.
 
 By default, Mercurial treats filenames as shell-style extended glob
 patterns.
 
 Alternate pattern notations must be specified explicitly.
 
 .. note::
   Patterns specified in ``.hgignore`` are not rooted.
   Please see :hg:`help hgignore` for details.
 
 To use a plain path name without any pattern matching, start it with
 ``path:``. These path names must completely match starting at the
 current repository root.
 
 To use an extended glob, start a name with ``glob:``. Globs are rooted
 at the current directory; a glob such as ``*.c`` will only match files
 in the current directory ending with ``.c``.
 
 The supported glob syntax extensions are ``**`` to match any string
 across path separators and ``{a,b}`` to mean "a or b".
 
 To use a Perl/Python regular expression, start a name with ``re:``.
 Regexp pattern matching is anchored at the root of the repository.
 
 To read name patterns from a file, use ``listfile:`` or ``listfile0:``.
 The latter expects null delimited patterns while the former expects line
 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
                  of the repository
   path:path:name a file or directory named "path:name"
 
 Glob examples::
 
   glob:*.c       any name ending in ".c" in the current directory
   *.c            any name ending in ".c" in the current directory
   **.c           any name ending in ".c" in any subdirectory of the
                  current directory including itself.
   foo/*.c        any name ending in ".c" in the directory foo
   foo/**.c       any name ending in ".c" in any subdirectory of foo
                  including itself.
 
 Regexp examples::
 
   re:.*\.c$      any name ending in ".c", anywhere in the repository
 
 File examples::
 
   listfile:list.txt  read list from list.txt with one file pattern per line
   listfile0:list.txt read list from list.txt with null byte delimiters
 
 See also :hg:`help filesets`.