Docs: Clarify the purpose of post date/time functions.

This changeset clarifies the purpose of these functions and make the documentation more accurate and flexible. Instead of referring to the "date the post was written," the functions and filter descriptions now refer to the "date of the post." This change accommodates scenarios where the displayed date might not strictly correspond to the writing date (e.g. scheduled posts, backdated posts, or content where the "date" represents something other than creation).

Props casiepa, audrasjb, SergeyBiryukov, Rarst, helen, azouamauriac, pbearne.
Fixes #51289.