Update conventions documentation for v4.0.0

Addresses the problem I noted in a comment on issue PyCQA#380

Author: peterjc

docs/error_codes.rst

@@ -9,16 +9,21 @@ Grouping
 Default conventions
 -------------------
 
-Not all error codes are checked for by default.  There are two
-conventions that may be used by pydocstyle: pep257 and numpy.
+Not all error codes are checked for by default.
 
-The pep257 convention, which is enabled by default in pydocstyle,
+The ``pep257`` convention, which is enabled by default in pydocstyle,
 checks for all of the above errors except for D203, D212, D213, D214,
 D215, D404, D405, D406, D407, D408, D409, D410, and D411 (as specified
 in `PEP257 <http://www.python.org/dev/peps/pep-0257/>`_).
 
-The numpy convention checks for all of the above errors except for
-D107, D203, D212, D213, D402, and D413.
+The ``numpy`` convention added in v2.0.0 supports the `numpydoc docstring
+<https://github.com/numpy/numpydoc>`_ standard. This checks all of of the
+errors except for D107, D203, D212, D213, D402, D413, D415, D416, and D417.
+
+The ``google`` convention added in v4.0.0 supports the `Google Python Style
+Guide <https://google.github.io/styleguide/pyguide.html>`_. This checks for
+all the errors except D203, D204, D213, D215, D400, D401, D404, D406, D407,
+D408, D409.
 
 These conventions may be specified using `--convention=<name>` when
 running pydocstyle from the command line or by specifying the