Use :func: etc in API docstrings

To cross-reference other Python objects use e.g.:

    :func:`ckan.logic.action.create.package_relationship_create()`

then Sphinx will format the link with the appropriate CSS and hyperlink it to the function.

Go through the action function docstrings and change references to other functions to use this.

Also `:class:`, `:mod:`, `:meth:` etc. see <http://sphinx-doc.org/domains.html#cross-referencing-python-objects>

Add this to the documentation guidelines.