Improve low-level backup documentation.

Improve low-level backup documentation.

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, also correcting some
errors introduced by 52f8a59dd953c6820baf153e97cf07d31b8ac1d6.
Updates to the 9.6 documentation are needed as well, but that
will need a separate patch as some things are different on that
version.

David Steele, reviewed by Robert Haas and Michael Paquier

Discussion: http://postgr.es/m/d4d951b9-89c0-6bc1-b6ff-d0b2dd5a8966@pgmasters.net