pgsql: Make each pg_stat_ view into it's own table in the documentation

Make each pg_stat_ view into it's own table in the documentation

This makes it easier to match a column name with the description of it,
and makes it possible to add more detailed documentation in the future.

This patch does not add that extra documentation at this point, only
the structure required for it.

Modeled on the changes already done to pg_stat_activity.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/da9ed7dafdc49e6571d056d92f2ef67858d68946

Modified Files
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doc/src/sgml/monitoring.sgml |  941 ++++++++++++++++++++++++++++++++++++++----
1 files changed, 859 insertions(+), 82 deletions(-)