interval: round values when spilling to months

Bruce Momjian <bruce@momjian.us>

Commit: 95ab1e0a9db321dd796344d526457016eada027f
Author: Bruce Momjian <bruce@momjian.us>
Date: 2021-08-03T16:10:29Z
Releases: 15.0
interval:  round values when spilling to months

Previously spilled units greater than months were truncated to months.
Also document the spill behavior.

Reported-by: Bryn Llewelly

Discussion: https://postgr.es/m/BDAE4B56-3337-45A2-AC8A-30593849D6C0@yugabyte.com

Backpatch-through: master

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