pubsub: Cliffhanger in documentation for Message type

[DenisPalnitsky]

Client
PubSub

Issue
Looks like part of the documentation is missing for Message type

Here is the exact location:

google-cloud-go/pubsub/message.go

Line 41 in 8b203b8

// these were properly processed by the server. If not,

Additional Context
I tried to find missing peace in git history without any success

[hongalex]

Thanks for filing this issue, opened a PR to fix that comment.