pgsql: Document how to prevent PostgreSQL itself from exhausting memory

Document how to prevent PostgreSQL itself from exhausting memory.

The existing documentation in Linux Memory Overcommit seemed to
assume that PostgreSQL itself could never be the problem, or at
least it didn't tell you what to do about it.

Per discussion with Craig Ringer and Kevin Grittner.

Branch
------
REL9_2_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/36bffe4ffa5a7492d78a889e7feb0838be5449f1

Modified Files
--------------
doc/src/sgml/runtime.sgml |   18 ++++++++++++++++--
1 files changed, 16 insertions(+), 2 deletions(-)