time-convert): Deprecate calls without an explicit FORM arg

time-convert): Deprecate calls without an explicit FORM arg

* lisp/subr.el (time-convert): Deprecate calls without an explicit FORM arg.

* doc/lispref/os.texi (Time Conversion): Adjust doc accordingly.

* lisp/calendar/time-date.el (days-to-time):
* lisp/emacs-lisp/timer.el (timer-next-integral-multiple-of-time):
* lisp/gnus/nnrss.el (nnrss-normalize-date):
* lisp/epa-ks.el (epa-ks--parse-buffer): Silence corresponding warnings.