Clarify comment style for module data members and class attributes

Author: kotfu

docs/doc_conventions.rst

@@ -153,6 +153,12 @@ have chosen to not use code as the source of instance attribute documentation.
 Instead, it is added manually to the documentation files in ``cmd2/docs/api``.
 See ``cmd2/docs/api/cmd.rst`` to see how to add documentation for an attribute.
 
+For module data members and class attributes, the ``autodoc`` extension allows
+documentation in a comment with special formatting (using a #: to start the
+comment instead of just #), or in a docstring after the definition. This project
+has standardized on the docstring after the definition approach. Do not use the
+specially formatted comment approach.
+
 
 Links to API Reference
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