pgsql: Docs: explain contrib/pg_stat_statements' handling of GC failure

Docs: explain contrib/pg_stat_statements' handling of GC failure.

Failure to perform garbage collection now has a user-visible effect, so
explain that and explain that reducing pgss_max is the way to prevent it.
Per gripe from Andrew Dunstan.

Branch
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REL9_5_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/d62359144da19297b13b2abf6c7d5d9220cfdf28

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