pgsql: Recommend include_realm=1 in docs

Recommend include_realm=1 in docs

As discussed, the default setting of include_realm=0 can be dangerous in
multi-realm environments because it is then impossible to differentiate
users with the same username but who are from two different realms.

Recommend include_realm=1 and note that the default setting may change
in a future version of PostgreSQL and therefore users may wish to
explicitly set include_realm to avoid issues while upgrading.

Branch
------
REL9_0_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/c981e599916c828fe3fe2e7c3cf92949f81ebcd3

Modified Files
--------------
doc/src/sgml/client-auth.sgml | 41 ++++++++++++++++++++++++++++++++---------
1 file changed, 32 insertions(+), 9 deletions(-)