Fixes and updates for Frames chapter in Emacs manual.

Fixes and updates for Frames chapter in Emacs manual.

* doc/emacs/files.texi (Visiting): `C-x 5 f' works on ttys too.

* doc/emacs/frames.texi (Frames): Rewrite introduction.
(Mouse Commands): Default for mouse-drag-copy-region is now t.
The mouse-save-then-kill-command does not copy to kill ring by
default.  Behavior of DEL is not special to mouse commands now.
(Mouse References): Document mouse-1-click-follows-link more
thoroughly.
(Menu Mouse Clicks): Move footnote to the main text and add xref
to Init Rebinding node.
(Mode Line Mouse): Mouse-3 on the mode-line does not bury buffer.

* lisp/window.el (display-buffer--special-action): Minor doc fix.