Make eldoc-echo-area-prefer-doc-buffer consider all frames

Make eldoc-echo-area-prefer-doc-buffer consider all frames

Previously, it considered only windows on 'visible' frames (according
to get-buffer-window).  This seemed correct at first, but it's not
much use for multiple TTY frames and not particularly reliable on GUI
frames either.  There's no reliable way to tell what is actually
visible, so it's best to assume that users setup frames that are
indeed visible when using this parameter.

* lisp/emacs-lisp/eldoc.el (eldoc--echo-area-prefer-doc-buffer-p):
Consider all frames.