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
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REL9_0_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/4fb505dff4fbf3b4661f20ec9cf00812b8ee9a2e

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