pgsql: Document that analyzing an empty table doesn't update the statis

Document that analyzing an empty table doesn't update the statistics.

Per a suggestion from Sergey Konoplev

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/db6de5b71ef2d91dde94fc0de8adf45372bc332d

Modified Files
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doc/src/sgml/ref/analyze.sgml |    6 ++++++
1 files changed, 6 insertions(+), 0 deletions(-)