Improve and extend documentation of ElDoc

Improve and extend documentation of ElDoc

* lisp/emacs-lisp/eldoc.el (eldoc-documentation-strategy)
(eldoc-documentation-enthusiast)
(eldoc-documentation-compose-eagerly)
(eldoc-documentation-compose, eldoc-documentation-default)
(eldoc-minibuffer-message, eldoc-idle-delay)
(eldoc-print-after-edit)
(eldoc-echo-area-display-truncation-message)
(eldoc-echo-area-use-multiline-p)
(eldoc-echo-area-prefer-doc-buffer): Doc string fixes.

* doc/emacs/emacs.texi (Top):
* doc/emacs/programs.texi (Documentation, Programming Language Doc):
Rename "Lisp Doc" to "Programming Language Doc", including in
parent menus.  All references changed.
(Programming Language Doc): Formerly "Lisp Doc".  Rewrite to not
be specific to Emacs Lisp.  Improve markup and wording.  Document
more commands and variables.
* doc/emacs/maintaining.texi (Maintaining): Improve indexing.