Add a godoc note about the API stability exception for mock types.

mocks/producer.go

@@ -10,6 +10,9 @@ import (
 // Before you can send messages to it's Input channel, you have to set expectations
 // so it knows how to handle the input. This way you can easily test success and
 // failure scenarios.
+//
+// NOTE: the SyncProducer type currently does not fall under the API stability
+// guarantee of Sarama as it is stiull considered experimental.
 type Producer struct {
 	l            sync.Mutex
 	expectations []*producerExpectation

mocks/sync_producer.go

@@ -9,6 +9,9 @@ import (
 // Before you can use it, you have to set expectations on the mock SyncProducer
 // to tell it how to handle calls to SendMessage, so you can easily test success
 // and failure scenarios.
+//
+// NOTE: the SyncProducer type currently does not fall under the API stability
+// guarantee of Sarama as it is stiull considered experimental.
 type SyncProducer struct {
 	l            sync.Mutex
 	t            ExpectationViolationReporter