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 <[email protected]>
Author: Robert Treat <[email protected]>
Co-authored-by: Tom Lane <[email protected]>
Reviewed-by: Dean Rasheed <[email protected]>
Discussion: https://postgr.es/m/2BD74F86-5B89-4AC1-8F13-23CED3546AC1@gmail.com
Backpatch-through: 13