bug#38535: 26.3; Optionally make recursive editing more noticeable

[Drew Adams]

> > Recursive editing is sometimes useful.
> > Its main disadvantage is that you might not
> > be aware that you've entered or exited a
> > recursive edit.
> 
> But there is minibuffer-depth-indicate-mode that shows the recursion
> depth in the minibuffer prompt when recursive minibuffers are enabled.

Unrelated.

Minibuffer recursion is indicated as you say.
It is not indicated by [...] in the mode line.

 "Minibuffer input is a special kind of
  recursive editing." (elisp) `Recursive Editing'

This is about a non-minibuffer recursive edit,
which is indicated only by a not-very-noticeable
[...] in the mode line.

Those two are handled very differently in terms
of UI and interaction.

For the same reason that we highlight the
minibuffer indication of minibuffer depth, minor
mode `rec-edit-mode' can highlight the [...]
brackets.  ("Can", because an option controls
whether the highlighting occurs.)