Document when GrpcNumChannelsOption applies

[dbolduc]

google-cloud-cpp/google/cloud/grpc_options.h

Lines 38 to 47 in 472f20a

	/**
	* The number of transport channels to create.
	*
	* gRPC limits the number of simultaneous calls in progress on a channel to
	* 100. Increasing the number of channels thus increases the number of
	* operations that can be in progress in parallel.
	*/
	struct GrpcNumChannelsOption {
	using Type = int;
	};

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; Document the connection pooling model in the Bigtable library. #1553)
• spanner::Connection
• pubsub::PublisherConnection
• pubsub::SubscriberConnection
• storage_experimental::DefaultGrpcClient() (i think)

[coryan]

Just fixing the existing comment seems good enough.