Stress difference of new and old ways to call `encode-time'

Stress difference of new and old ways to call `encode-time'

* doc/lispref/os.texi (Time Conversion): Add a warning that blind
changing of code calling `encode-time' to use single list instead of
multiple values may cause deferred bugs since it is common to use nil
for ignored arguments such as DST in the old calling convention.
* src/timefns.c (encode-time): Mention the warning added to the elisp
reference in the docstring.

Refactoring related to `encode-time' caused (bug#54731), so it is better
to make apparent the difference between the recommended and the
obsolescent ways to call the function.  More details concerning the
purpose and limitations of the DST field are added after discussion with
Paul Eggert in (bug#54764).