bug#30085: 25.2: Documentation for cua-rectangle-mark-mode

[Boruch Baum]

I realize that some elisp code that I wrote for another purpose might be
useful for this mode.

  https://github.com/Boruch-Baum/swiper

One of the elements of `cua-rectangle-mark-mode' is a one-line cheat-sheet
style message of default keybindings of basic features, itself by
default bound to `C-?'.

In my referenced link above, I provide for `swiper' two features:

1) Cheat-sheet style messages that appear above the minibufer.

1.1) Keybinding for the messages (`M'?' by default) turns on the
     display, scrolls to the next message, and turns off the display.

1.2) The default message(s) may be supplemented or replaced by messages
     set by the user.

1.3) The message format includes an easy method to ensure that changing
     a keybinding does not require going back and editing the message.

2) Visual feedback of the current state of the mode. In the case of
   swiper, this is a single-line indication of case-fold status, regex
   method, currently selected action, and auto status. This feedback
   line appears just above the minibuffer.

The elisp code should be usable for other "complicated" emacs commands
without much of any modification. In the case of `swiper', it was
necessary to make a change to function `ivy-read' because of its
recursive nature and its use of unwind.

In the case of `swiper', a pull request has been made to the developer,
and is waiting for feedback.

An example of another candidate for this proposed feature would be
`isearch-forward'.

I would like to eventually offer it in generic form to emacs, but as for
now it hasn't even gotten approved by `swiper'.

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