Improve documentation in setting FSM

I have applied the following patch suggesting people look at the last
few lines of a database-wide VACUUM VERBOSE to determine if the current
setting is appropriate. Also mention the NOTICE that is printed if it
is too low.

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