Add API documentation for MessageReader

[AArnott]

I'm implementing my own MessageReader and MessageWriter. The MessageReader.onPartialMessage isn't documented and I'm having a difficult time figuring out what it's for. I may have to go digging into the source code to figure it out.
Can API docs be added?

[dbaeumer]

Yes, of course. If I recall it correctly I added this a long time ago based on your suggestion. If signals the fact the a message after some timeout did not fully make it to the receiver.

[AArnott]

Funny. I don't remember making that suggestion, but thanks for taking the PR.