Added documentation about caching, reliability

I have added a section to the top of the WAL docs explaining caching and
reliability issues:

    http://candle.pha.pa.us/main/writings/pgsql/sgml/reliability.html

I also renamed the chapter "Reilability" rather than "WAL."

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  Bruce Momjian                        |  http://candle.pha.pa.us
  pgman@candle.pha.pa.us               |  (610) 359-1001
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