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_2_STABLE

Modified Files:
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    pgsql/doc/src/sgml:
        datatype.sgml (r1.181.2.4 -> r1.181.2.5)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/datatype.sgml?r1=1.181.2.4&r2=1.181.2.5)