Fix settings.md strftime URL and clarify the representation "a Python strftime format string"

[t-ubukata]

The [strftime] link reference definition defines
https://docs.python.org/3/library/time.html#time.strftime
It's the URL of time.strftime(format[, t]), but this documentation is different from strftime() and strptime() Behavior,
https://docs.python.org/3/library/datetime.html#strftime-strptime-behavior
Former doen't take %f, latter takes, and actually datetime.now().strftime() takes %f and time.strftime() doesn't take %f in CPython 3.10.12.
Latter says "date, datetime, and time objects all support a strftime(format) method, to create a string representing the time under the control of an explicit format string.".
It looks like a Python issue, but it's better to clarify the representation "a Python strftime format string" means what.
For example, it means time.strftime() in TIME_FORMAT block, and means datetime.datetime.strftime() in DATETIME_FORMAT block.
One option is to divide this definition into two URLs and clarify the representation "a Python strftime format string" means what.