Document encode-time caveats

Document encode-time caveats

* doc/lispref/os.texi (Time of Day, Time Conversion):
Move the warnings about DST being -1 to closer to where DST is
discussed, and reword and improve the discussions and warnings.
Be more precise about years before 1969 (possible west of UTC) vs the
Epoch.  Mention some problems due to leap seconds, leap years,
daylight saving transitions, and time zone changes.  Modernize
discussion of OS timestamp range.  Prefer secular ‘BCE’ to religious
‘BC’.  Omit discussion of decoded-time-add and make-decoded-time, as
they are in a library and are not always available; instead, mention
the library.  Warn about common mistakes when doing simple date
arithmetic.
* src/timefns.c (Fencode_time): In doc string, mention date
arithmetic and tighten up the wording a bit.