Add documentation for renderer heading when TOC enabled

ref: #251

Author: lepture

docs/advanced.rst

@@ -37,6 +37,7 @@ Here is a a list of available renderer functions::
     # block level
     paragraph(self, text)
     heading(self, text, level)
+    heading(self, text, level, tid)  # when TOC directive is enabled
     thematic_break(self)
     block_text(self, text)
     block_code(self, code, info=None)

docs/directives.rst

@@ -75,6 +75,17 @@ TOC plugin::
         plugins=[DirectiveToc()]
     )
 
+If **TOC** directive is enabled, the ``heading`` method of renderer will accept
+one more paramter::
+
+    def heading(self, text, level):
+        # without TOC directive
+        return ''
+
+    def heading(self, text, level, tid):
+        # with TOC directive
+        return ''
+
 Include
 -------