Document `:module:` option in `domains.rst`

[not-my-profile]

The module & class content directives accept a :module: option, for example:

.. function:: foo()
   :module: bar

Currently this is only shown once in the directives section of the reStructuredText Primer. I think it would make sense to additionally document it on the Domains page, because that is where the py:module and py:currentmodule module directives are documented.