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
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/5d72f85d4ca8e0a56836ea48576ff8f684f7dce7

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