doc: add missing sentences in a function comment

Refs: #2374

mocks/consumer.go

@@ -204,8 +204,10 @@ func (c *Consumer) SetTopicMetadata(metadata map[string][]int32) {
 // The registered PartitionConsumer will be returned, so you can set expectations
 // on it using method chaining. Once a topic/partition is registered, you are
 // expected to start consuming it using ConsumePartition. If that doesn't happen,
-// an error will be written to the error reporter once the mock consumer is closed. It will
-// also expect that the
+// an error will be written to the error reporter once the mock consumer is closed. It also expects
+// that the message and error channels be written with YieldMessage and YieldError accordingly,
+// and be fully consumed once the mock consumer is closed if ExpectMessagesDrainedOnClose or
+// ExpectErrorsDrainedOnClose have been called.
 func (c *Consumer) ExpectConsumePartition(topic string, partition int32, offset int64) *PartitionConsumer {
 	c.l.Lock()
 	defer c.l.Unlock()