Improve documentation of 'date-to-time' and 'parse-time-string'

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)