Provide documentation for lcb_set_bootstrap_callback()

Change-Id: If1df67afed11cf55cdf412971b69c3e5b5a6c755
Reviewed-on: http://review.couchbase.org/38535
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Brett Lawson <brett19@gmail.com>