pgsql: Fix and clarify information schema interval_precision fields

Fix and clarify information schema interval_precision fields

The fields were previously wrongly typed as character_data; change to
cardinal_number.  Update the documentation and the implementation to
show more clearly that this applies to a feature not available in
PostgreSQL, rather than just not yet being implemented in the
information schema.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/3315020a091f64c8d08c3b32a2abd46431dcf857

Modified Files
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doc/src/sgml/information_schema.sgml       |   37 +++++++++++++++++++--------
src/backend/catalog/information_schema.sql |   16 ++++++------
2 files changed, 34 insertions(+), 19 deletions(-)