Fix assorted misstatements and poor wording in the descriptions of the I/O

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.