'gapi' has an undocumented 'cores' section

[filmil]

This is about https://fusesoc.readthedocs.io/en/stable/user/build_system/generators.html.

I am aware of the TODO there, but perhaps a TODO also needs an issue.

The generators API ('gapi') doc seems out of date. It is no longer explaining all the gapi sections, specifically the cores section and its contents and meaning.

Please consider adding some documentation there.