Include interactive-only information in describe-function output

Include interactive-only information in describe-function output

* lisp/help-fns.el (help-fns--interactive-only): New function.
(help-fns-describe-function-functions): Add the above function.

* lisp/simple.el (beginning-of-buffer, end-of-buffer, insert-buffer)
(next-line, previous-line): Remove hand-written interactive-only
information from doc strings, it is auto-generated now.

* lisp/bookmark.el (bookmark-write):
* lisp/epa-mail.el (epa-mail-decrypt, epa-mail-verify, epa-mail-sign)
(epa-mail-import-keys): Mark interactive-only,
and remove hand-written interactive-only information from doc strings.

* lisp/epa.el (epa-decrypt-armor-in-region, epa-verify-region)
(epa-verify-cleartext-in-region, epa-sign-region, epa-encrypt-region):
* lisp/files.el (not-modified):
* lisp/simple.el (mark-whole-buffer): Mark interactive-only.

* doc/lispref/commands.texi (Defining Commands):
Mention that interactive-only also affects describe-function.

* etc/NEWS: Mention this.