add a Documentation() class to the API, which will be used in the future for all kind of docstrings. Also add a documentation method on BaseDefinition that returns a Documentation object. Deprecate at the same time its doc and raw_doc functions

test/test_api/test_call_signatures.py

@@ -193,7 +193,7 @@ def test_signature_is_definition():
 
     # Now compare all the attributes that a CallSignature must also have.
     for attr_name in dir(definition):
-        dont_scan = ['defined_names', 'line_nr', 'start_pos']
+        dont_scan = ['defined_names', 'line_nr', 'start_pos', 'documentation', 'doc']
         if attr_name.startswith('_') or attr_name in dont_scan:
             continue
         attribute = getattr(definition, attr_name)