Document time-convert FORM argument as mandatory

Document time-convert FORM argument as mandatory

This is a followup to commit b70369c557 of 2022-08-05
"time-convert): Deprecate calls without an explicit FORM arg".

* doc/lispref/os.texi (Time Conversion):
* src/timefns.c (Ftime_convert): Describe FORM argument as required
as per the advertised calling convention.