Better document v3 API

This reworks the entire api3 file (but not changing anything else in the
code) by reorgaizing the sections of the file. It also moves the more
low level buffer stuff to kvbuf.h.

Change-Id: I765b18a410c2591edecd61040e96c1d6fb16b4b5
Reviewed-on: http://review.couchbase.org/38927
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Brett Lawson <brett19@gmail.com>