Document the 'expand' API GET parameter

'expand' can be used to retrieve related fields in one query within the API. This needs documenting.

There's a non-documented API GET parameter that can be useful to you right now:

http://patchwork.freedesktop.org/api/1.0/projects/intel-gfx/series/?ordering=-last_updated&related=expand

The key is the related=expand. It will "expand" the related fields, ie the primary keys will become a serialization of the object pointed at by that primary key.

I haven't documented this API yet as I'd like to ensure it makes the number of SQL queries still O(1) (ie not depending on the number of series returned). This is true on the series entry point (it even has test coverage!), but not tested for all the "list" entry points.