Show large docs in help buffer instead of echo are by default

Show large docs in help buffer instead of echo are by default

* eglot.el (eglot--managed-mode): Add and remove from eglot--eldoc-message
(eglot--eldoc-hint, eglot--help-buffer): New helpers.
(eglot-eldoc-extra-buffer)
(eglot-auto-display-eldoc-extra-buffer): New defcustoms.
(eglot--eldoc-message): New helper.
(eglot-eldoc-function): Set eglot--eldoc-hint.
(eglot-help-at-point): Use new helpers.
(eglot-eldoc-extra-buffer-if-too-large): New predicate.

GitHub-reference: per https://github.com/joaotavora/eglot/issues/198