Adjust Eldoc documentation after Eli's review

Adjust Eldoc documentation after Eli's review

* etc/NEWS (Eldoc): Adjust paragraphs.

* lisp/emacs-lisp/eldoc.el (eldoc-prefer-doc-buffer): Adjust
docstring.
(eldoc--enthusiasm-curbing-timer, eldoc-documentation-strategy)
(eldoc-documentation-functions): Adjust docstring.
(eldoc--handle-docs): Adjust comments.
(eldoc--documentation-compose-1): New helper.
(eldoc-documentation-compose)
(eldoc-documentation-compose-eagerly): Use it.
(eldoc-print-current-symbol-info): Adjust comments.