pgsql: doc: Document error handling in PGTYPESnumeric_to_long

doc: Document error handling in PGTYPESnumeric_to_long

The documentation for PGTYPESnumeric_to_long only mentioned errno
being set to indicate overflow but the code also sets errno when
underflow happens.

Reported-by: Aidar Imamov <a.imamov@postgrespro.ru>
Discussion: /messages/by-id/eebf0ad50ad4321d65d2d64dd6b7f17d@postgrespro.ru

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/68174679fea2d514e9281fddd826abaccfac5bf7

Modified Files
--------------
doc/src/sgml/ecpg.sgml | 5 +++--
1 file changed, 3 insertions(+), 2 deletions(-)