Clarify backref documentation

Fixes #6

src/vmod_re.vcc

@@ -99,13 +99,13 @@ matched. When a match with the ``match`` or ``match_body`` method
 succeeds, then a captured string can be obtained from the ``backref``
 method.
 
-Calls to the ``backref`` method refer back to the most recent
-successful call to ``match`` or ``match_body`` for the same object in
-the same task scope; that is, in the same client or backend
-context. For example if ``match`` is called for an object in one of
-the ``vcl_backend_*`` subroutines and returns ``true``, then
-subsequent calls to ``backref`` in the same backend scope extract
-substrings from the matched substring.
+Calls to the ``backref`` method refer back to the most recent call to ``match``
+or ``match_body`` for the same object in the same task scope; that is, in the
+same client or backend context. For example if ``match`` is called for an object
+in one of the ``vcl_backend_*`` subroutines and returns ``true``, then
+subsequent calls to ``backref`` in the same backend scope extract substrings
+from the matched substring. For an unsuccessful match, all back references are
+cleared.
 
 By setting the ``asfilter`` parameter to true, a regex object can also
 be configured to add a filter for performing substitutions on