Add documentations related to new APIs of r313761

PR: 216850 216851 216852 216856 216857 216858
Submitted by: sgk@troutmask.apl.washington.edu
Reported by: sgk@troutmask.apl.washington.edu
Reviewed by: bde emaste hselasky
Approved by: bde emaste hselasky
Differential Revision: https://reviews.freebsd.org/D9491