Link concrete entry classes into usage docs. (#5512)
Link concrete entry classes into usage docs. (#5512)

Closes #4118.

Note that those API docs won't be as useful as they could be without
either making '_BaseEntry' public (see #5511) or going to extra effort
to propagate the signature docs for '__init__'.
jonparrott/google-cloud-python,tswast/google-cloud-python,GoogleCloudPlatform/gcloud-python,dhermes/gcloud-python,GoogleCloudPlatform/gcloud-python,tswast/google-cloud-python,tswast/google-cloud-python,dhermes/google-cloud-python,googleapis/google-cloud-python,dhermes/google-cloud-python,jonparrott/gcloud-python,dhermes/gcloud-python,tseaver/gcloud-python,dhermes/google-cloud-python,jonparrott/google-cloud-python,tseaver/gcloud-python,tseaver/google-cloud-python,tseaver/google-cloud-python,googleapis/google-cloud-python,tseaver/google-cloud-python,jonparrott/gcloud-python