Add an UPDATING entry and warning about the change in r274807 to help users
transition to the new behavior.

Discussed with:	jmallett
Sponsored by:	DARPA, AFRL


git-svn-id: svn+ssh://svn.freebsd.org/base/head@275839 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f