Enhance API documentation

- Undocument older create_st fields
- Document iops integration APIs with libev, libevent, and libuv

Change-Id: Ia1d889b2cfc406fde015652062b90b2640070dbf
Reviewed-on: http://review.couchbase.org/38331
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Brett Lawson <brett19@gmail.com>