Schema doc generation picks up an extraneous "Struct" category

[kozbo]

see http://ga4gh-schemas.readthedocs.io/en/latest/schemas/struct.proto.html

I am guessing that our doc generation code is picking up the
import "google/protobuf/struct.proto"
and presenting it as part of our schema.

[david4096]

We use Google's ListValue and so some documentation to work with these data are provided. We might do a better job of separating documenting our data model and provide links out to Google where needed.