Introduce eldoc-display-functions

Introduce eldoc-display-functions

See bug#43609.

* lisp/emacs-lisp/eldoc.el (eldoc--request-state): Add comment.
(eldoc--last-request-state): No longer buffer-local.
(eldoc--request-docs-p): Delete.
(eldoc-display-functions): New user variable.
(eldoc--doc-buffer-docs): New variable.
(eldoc-display-message-p): Rework.
(eldoc--format-doc-buffer): Rework from eldoc--handle-docs.
(eldoc-display-in-echo-area, eldoc-display-in-buffer): New
user-visible function.
(eldoc--invoke-strategy): Take INTERACTIVE arg.
Invoke eldoc-display-in-buffer
(eldoc-print-current-symbol-info): Simplify.
(Version): Bump to 1.11.0

* etc/NEWS: Mention eldoc-display-functions.