pubsub: clarify extension documentation

Change-Id: I9e886a95bba1338145e6b1dd80616a0442ffec4c
Reviewed-on: https://code-review.googlesource.com/c/35094
Reviewed-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Michael Darakananda <pongad@google.com>

pubsub/doc.go

@@ -117,6 +117,17 @@ minutes, every 3 minutes. Suppose the application crashes 5 seconds after the
 library sends such an extension: the Pub/Sub server would wait the remaining
 2m55s before re-sending the messages out to other subscribers.
 
+Please note that the client library does not use the subscription's AckDeadline
+by default. To enforce the subscription AckDeadline, set MaxExtension to
+to the subscription's AckDeadline:
+
+	cfg, err := sub.Config(ctx)
+	if err != nil {
+		// TODO handle err
+	}
+
+	sub.ReceiveSettings.MaxExtension = cfg.AckDeadline
+
 Slow Message Processing
 
 For use cases where message processing exceeds 30 minutes, we recommend using

pubsub/subscription.go

@@ -203,7 +203,8 @@ type ReceiveSettings struct {
 	//
 	// The Subscription will automatically extend the ack deadline of all
 	// fetched Messages up to the duration specified. Automatic deadline
-	// extension may be disabled by specifying a duration less than 0.
+	// extension beyond the initial receipt may be disabled by specifying a
+	// duration less than 0.
 	MaxExtension time.Duration
 
 	// MaxOutstandingMessages is the maximum number of unprocessed messages
@@ -436,7 +437,8 @@ var errReceiveInProgress = errors.New("pubsub: Receive already in progress for t
 // The context passed to f will be canceled when ctx is Done or there is a
 // fatal service error.
 //
-// Receive will automatically extend the ack deadline of all fetched Messages up to the
+// Receive will send an ack deadline extension on message receipt, then
+// automatically extend the ack deadline of all fetched Messages up to the
 // period specified by s.ReceiveSettings.MaxExtension.
 //
 // Each Subscription may have only one invocation of Receive active at a time.