pgsql: Mention extra_float_digits in floating point docs

Mention extra_float_digits in floating point docs

Make it easier for readers of the FP docs to find out about possibly
truncated values.

Per complaint from Tom Duffey in message
F0E0F874-C86F-48D1-AA2A-0C5365BF5118@trillitech.com

Author: Albe Laurenz
Reviewed by: Abhijit Menon-Sen

Branch
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REL8_4_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/e9ac1b8775c49a41374d85463f56aed73d512f25

Modified Files
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doc/src/sgml/config.sgml   |    1 +
doc/src/sgml/datatype.sgml |   11 +++++++++++
2 files changed, 12 insertions(+)