Improve inline documentation for several 'last_changed'-related functions introduced in [27101] and [27102].

Functions include:
* `get_taxonomy_last_changed()`
* `set_taxonomy_last_changed()`
* `post_taxonomy_is_fresh()`
* `taxonomy_hierarchy_is_fresh()`

Fixes #22526, #14485.


git-svn-id: https://develop.svn.wordpress.org/trunk@27141 602fd350-edb4-49c9-b593-d223f7449a82