Our documentation hasn't really caught up with the fact that
non-exclusive backups can now be taken using pg_start_backup and
pg_stop_backup even on standbys.  Update.

David Steele, reviewed by Robert Haas and Michael Paquier

Discussion: http://postgr.es/m/f349b834-1443-ebf0-3c2a-965f944004d7@pgmasters.net