[REGAPIC] Rest client docs include yard xref links to non-rest client classes

[dazuma]

The code that generates yard links for cross-reference syntax (located in gapic-generator/lib/gapic/formatting_utils.rb) assumes GRPC client classes. So cross-references in REST client docs will yield broken yard links to where the GRPC class would have been. Specifically, I think yard_link_for_entity needs to know whether the target is a REST or GRPC client, and generate client class names accordingly.