Improve documentation for year-zero issues

Improve documentation for year-zero issues

* doc/emacs/calendar.texi (Calendar Systems)
* doc/lispref/os.texi (Time Conversion):
Prefer "BC" to "B.C." since the documentation generally uses "BC".
* doc/misc/emacs-mime.texi (time-date):
* lisp/calendar/time-date.el (date-to-day, time-to-days):
In the doc string, state the day origin more clearly, and more
consistently with the rest of the documentation.
* src/timefns.c (Fdecode_time): State the year origin in
the doc string.