Functions missing from REST api docs

[ivankelly]

http://pulsar.apache.org/en/admin-rest-api/

There's no mention of the functions endpoints.

[ivankelly]

Swagger was failing because the function endpoint was returning some protobuf. This protobuf needs to be refactored out of the API, and then swagger should work.

[sijie]

The docs of functions, sinks and sources are all added.

[sijie]

The docs of functions, sinks and sources are all added.