Fix some bugs in netbuf:

- start_flush: Returned IOV count off by one
- end_flush_2: Set PDU offset on partial flush
- document pdu callback better

Change-Id: I3e172dbd63c5a68721256857681ebca397ab3bf4
Reviewed-on: http://review.couchbase.org/35868
Reviewed-by: Brett Lawson <brett19@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>