pgsql: Clarify terminology standalone backend vs. single-user mode

Clarify terminology standalone backend vs. single-user mode

Most of the documentation uses "single-user mode", so use that in the
code as well.  Adjust the documentation to match the new error message
wording.  Also add a documentation index entry for "single-user mode".

Based-on-patch-by: Jeff Janes <jeff.janes@gmail.com>

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/7dfd5cd21c0091e467b16b31a10e20bbedd0a836

Modified Files
--------------
doc/src/sgml/maintenance.sgml       |   10 +++++-----
doc/src/sgml/ref/postgres-ref.sgml  |    4 ++++
src/backend/access/transam/varsup.c |    6 +++---
3 files changed, 12 insertions(+), 8 deletions(-)