Update `wp_allow_comment()` and `pre_comment_approved` docs with possible `trash` return value

[jeremyfelt]

In wp_allow_comment(), if wp_blacklist_check() is true and EMPTY_TRASH_DAYS is defined and truthy, then trash is returned for the value of $approved.

The docs for wp_allow_comment() and the pre_comment_approved filter should be updated to reflect this.

[SergeyBiryukov]

In 47286:

Docs: Update @return tag for wp_allow_comment() with trash as a possible return value.

Props oakesjosh, jeremyfelt.
Fixes #49206.