pgsql: Fix assorted misstatements and poor wording in the descriptions

Log Message:
-----------
Fix assorted misstatements and poor wording in the descriptions of the I/O
formats for geometric types. Per bug #5536 from Jon Strait, and my own
testing.

Back-patch to all supported branches, since this doco has been wrong right
along -- we certainly haven't changed the I/O behavior of these types in
many years.

Tags:
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REL8_4_STABLE

Modified Files:
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pgsql/doc/src/sgml:
datatype.sgml (r1.239.2.1 -> r1.239.2.2)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/datatype.sgml?r1=1.239.2.1&r2=1.239.2.2)