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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master

Details
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http://git.postgresql.org/pg/commitdiff/35738b5906bb80bc4bb77ce1adea7dfbd3f64274

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