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_1_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/f6956eb74edcdf21339f9d200ef63c4015a5f4cb

Modified Files
--------------
doc/src/sgml/runtime.sgml |   20 +++++++++++++++++---
1 files changed, 17 insertions(+), 3 deletions(-)