Document when `GrpcNumChannelsOption` applies

[dbolduc]

https://github.com/googleapis/google-cloud-cpp/blob/472f20a660310973191fcb99b7fbca81ec843a47/google/cloud/grpc_options.h#L38-L47

This option only applies in certain libraries (the handwritten ones). We should make this clear.

This means documenting the option:

@note This option only applies to a few libraries. Check the service's `*Connection` documentation for details.

... and documenting the connection pool for the Connections that use it.

• bigtable::DataConnection (currently under development; #1553)
• spanner::Connection
• pubsub::PublisherConnection
• pubsub::SubscriberConnection
• storage_experimental::DefaultGrpcClient() (i think)

[coryan]

Just fixing the existing comment seems good enough.