Improve documentation of 'date-to-time' and 'parse-time-string'
* doc/lispref/os.texi (Time Parsing): Document 'parse-time-string', and refer to it for the description of the argument of 'date-to-time'. * lisp/calendar/time-date.el (date-to-time): Refer in the doc string to 'parse-time-string' for more information about the format of the DATE argument. (Bug#34303)