allow automodapi to document individual items #265

This was discussed in #219 as item 3. From @mdboom:

I want to document some classes that aren't listed in the module's __all__ but are returned from functions in that module. It's helpful to know how the class works, but instantiating the class is a private implementation detail. Having a way of listing additional module members might do the trick. They wouldn't have to be displayed in a separate list. This is a pretty low-priority concern relative to the others. In the end, I've just added the class to __all__ as a workaround for now.

This could be implemented as an option to automodapi, but the catch is that we want a way to make it apparent that the item is not a public item that is importable. It's not clear what the best way is to do this, though...