ElDoc: add docstrings and minor refactoring

ElDoc: add docstrings and minor refactoring

* lisp/emacs-lisp/eldoc.el (eldoc-edit-message-commands): Add
docstring.
(turn-on-eldoc-mode): Fix capitalization.
(eldoc--supported-p): Add docstring.
(eldoc-schedule-timer): Add docstring and use
'eldoc--supported-p'.
(eldoc-message): Add docstring and make calling convention
clearer.
(eldoc--message-command-p):
(eldoc-pre-command-refresh-echo-area):
(eldoc-display-message-p):
(eldoc-display-message-no-interference-p):
(eldoc-print-current-symbol-info):
(eldoc-docstring-format-sym-doc):
(eldoc-add-command, eldoc-add-command-completions):
(eldoc-remove-command, eldoc-remove-command-completions):
Add docstring.  (Bug#27230)