bug#36979: Calendar: mention how to copy date

[積丹尼 Dan Jacobson]

(info "(emacs) To Other Calendar")'s
p C             calendar-chinese-print-date
p a             calendar-astro-print-day-number
p b             calendar-bahai-print-date
p c             calendar-iso-print-date...
are great, except you need to mention:

If you not only want to view the date in the minibuffer, but also want
to copy it, then also use (view-echo-area-messages). It is bound to C-h e...