[3.13] gh-148529: Minor improvements of the struct module documentation (GH-148565) (GH-149072)

* Document that 's' and 'p' accept bytes and bytearray.
* Fix some footnotes.
* Clarify that "string" is a byte string.
* Fix the module docstring.
(cherry picked from commit 3e5a3cb2bd222f97f793b01bc1c0f7bb62aefc31)