docs/ure: Emphasize not supported features more.

Plus, additional descriptions/formatting.

docs/library/ure.rst

@@ -15,8 +15,9 @@ Supported operators are:
 ``'.'``
    Match any character.
 
-``'[]'``
-   Match set of characters. Individual characters and ranges are supported.
+``'[...]'``
+   Match set of characters. Individual characters and ranges are supported,
+   including negated sets (e.g. ``[^a-c]``).
 
 ``'^'``
 
@@ -36,12 +37,13 @@ Supported operators are:
 
 ``'|'``
 
-``'()'``
+``'(...)'``
    Grouping. Each group is capturing (a substring it captures can be accessed
    with `match.group()` method).
 
-Counted repetitions (``{m,n}``), more advanced assertions, named groups,
-etc. are not supported.
+**NOT SUPPORTED**: Counted repetitions (``{m,n}``), more advanced assertions
+(``\b``, ``\B``), named groups (``(?P<name>...)``), non-capturing groups
+(``(?:...)``), etc.
 
 
 Functions