pgsql: Docs: add another example of creating a range type.

Docs: add another example of creating a range type.

The "floatrange" example is a bit too simple because float8mi can be
used without any additional type conversion.  Add an example that does
have to account for that, and do some minor other wordsmithing.

Branch
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REL9_5_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/1dc6f557e7020269436cbf8a66e68bc6e66def0c

Modified Files
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doc/src/sgml/rangetypes.sgml |   53 ++++++++++++++++++++++++++++--------------
1 file changed, 35 insertions(+), 18 deletions(-)