Merge branch 'entry-point' into 'master'

Clarify what the X in the entry points does

*Description of changes*

Previously the `X` wasn't further explained in the documentation on how to
register a plugin. This plugs the hole at least for checking plugins.

*Related to:*  #183

See merge request !93

docs/source/plugin-development/registering-plugins.rst

@@ -80,7 +80,7 @@ Note specifically these lines:
         # snip ...
     )
 
-We tell setuptools to register our entry point "X" inside the specific
+We tell setuptools to register our entry point ``X`` inside the specific
 grouping of entry-points that flake8 should look in.
 
 |Flake8| presently looks at three groups:
@@ -111,6 +111,10 @@ like:
         # snip ...
     )
 
+The ``X`` in checking plugins define what error codes it is going to report.
+So if the plugin reports just the error code ``A101`` it would be that. If it
+reports also ``A102`` it must be set to ``A10``.
+
 
 .. _Entry Points:
     https://pythonhosted.org/setuptools/pkg_resources.html#entry-points