Support displaying when lisp subroutine/variable was introduced.

[cireu]

Emacs 27 support displaying when a lisp subroutine/variable was intorduced.

For example, (describe-function 'rx)

Get

rx is an autoloaded Lisp macro in ‘rx.el’.

(rx &rest REGEXPS)

  Probably introduced at or before Emacs version 21.1.

It will be good if helpful.el support this ;)