pgsql: docs: clarify infinite range values from data-type infinities

docs: clarify infinite range values from data-type infinities

The previous docs referenced these distinct ideas confusingly.

Reported-by: Eugen Konkov

Discussion: /messages/by-id/376945611.20191026161529@yandex.ru

Backpatch-through: 9.4

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/650692a18d29c44c8019545c02ba42c3efd2c964

Modified Files
--------------
doc/src/sgml/rangetypes.sgml | 37 +++++++++++++++++--------------------
1 file changed, 17 insertions(+), 20 deletions(-)