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: https://postgr.es/m/
eebf0ad50ad4321d65d2d64dd6b7f17d@postgrespro.ru