Requested feature overview description.
Have the API return the pool IDs for a post.

This feature was supposedly added a few years back, but I don't see it in the API responses.

Why would it be useful?
It'd allow third-party tools to show the pool a post belongs to.

What part(s) of the site page(s) are affected?
/post/index.xml
/post/show/?.xml
(wherever a post object is returned by the API)