pgsql: Doc: improve documentation about width_bucket().

Doc: improve documentation about width_bucket().

Specify whether the bucket bounds are inclusive or exclusive,
and improve some other vague language. Explain the behavior that
occurs when the "low" bound is greater than the "high" bound.
Make width_bucket_numeric's comment more like that for
width_bucket_float8, in particular noting that infinite
bounds are rejected (since they became possible in v14).

Reported-by: Ben Peachey Higdon <bpeacheyhigdon(at)gmail(dot)com>
Author: Robert Treat <rob(at)xzilla(dot)net>
Co-authored-by: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Reviewed-by: Dean Rasheed <dean(dot)a(dot)rasheed(at)gmail(dot)com>
Discussion: https://postgr.es/m/2BD74F86-5B89-4AC1-8F13-23CED3546AC1@gmail.com
Backpatch-through: 13

Branch
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REL_16_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/7a93bd5986e4b741fc3fdd8406431c77161c6d12

Modified Files
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doc/src/sgml/func.sgml | 18 ++++++++++++++----
src/backend/utils/adt/float.c | 4 ++--
src/backend/utils/adt/numeric.c | 7 ++++---
3 files changed, 20 insertions(+), 9 deletions(-)