pgsql: Doc: correct overambitious precision claim for contrib/seg.

Doc: correct overambitious precision claim for contrib/seg.

seg.sgml claims that a seg can store 7 significant digits, but this
is nonsense. The underlying float4 data type is not (quite) that
precise, and seg_out() intentionally rounds off to at most FLT_DIG
decimal places. With IEEE float arithmetic, FLT_DIG will be defined
as 6, or at least that's the case on all platforms I know of.

AFAICT this thinko goes clear back to the introduction of seg
(commit a3694b420), but it wasn't questioned until now.

Bug: #19744
Reported-by: Ke <kehan5800(at)gmail(dot)com>
Author: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Discussion: https://postgr.es/m/19744-8c700582bbb1d075@postgresql.org
Backpatch-through: 14

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/98c41fed6fce927539400bc02974b9cb3184befc

Modified Files
--------------
doc/src/sgml/seg.sgml | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)