Clarify usage of `match()` for regex validator

- Closes #277

colander/__init__.py

@@ -320,6 +320,8 @@ class Regex(object):
         uses Python's :py:func:`re.match`, which only matches at the beginning
         of the string and not at the beginning of each line. To match the
         entire string, enclose the regular expression with ``^`` and ``$``.
+        If ``msg`` is supplied, it will be the error message to be used;
+        otherwise, defaults to 'String does not match expected pattern'.
 
         The ``regex`` expression behaviour can be modified by specifying
         any ``flags`` value taken by ``re.compile``.