Improve RefBase documentation, especially for clients.

Add basic interface documentation to RefBase.h.

Much, but not all, of this is cut-and-pasted from an email message
from Mathias Agopian. The rest is reconstructed from the code.

Delete some, now redundant, text from Refbase.cpp, and add a bit
more about the implementation strategy.

Some minor fixes to internal comments.

Bug: 30292291
Change-Id: I56518ae5553bc6de0cc2331778e7fcf2e6c4fd87