pubsub: clarify doc for ReceiveSettings.NumGoroutines

Fixes #712.

Change-Id: I0c28081933c35ed3f2baf1617f39a2f92c7fd552
Reviewed-on: https://code-review.googlesource.com/15690
Reviewed-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Michael Darakananda <pongad@google.com>

pubsub/subscription.go

@@ -152,6 +152,12 @@ type ReceiveSettings struct {
 	// NumGoroutines is the number of goroutines Receive will spawn to pull
 	// messages concurrently. If NumGoroutines is less than 1, it will be treated
 	// as if it were DefaultReceiveSettings.NumGoroutines.
+	//
+	// NumGoroutines does not limit the number of messages that can be processed
+	// concurrently. Even with one goroutine, many messages might be processed at
+	// once, because that goroutine may continually receive messages and invoke the
+	// function passed to Receive on them. To limit the number of messages being
+	// processed concurrently, set MaxOutstandingMessages.
 	NumGoroutines int
 }