bug#31790: 27.0; Mention Isearch key bindings of Isedarch variables that are bound by default in Isearch

[Drew Adams]

> And having that in a doc string of a non-user variable will make it
> easier to pick up?

Yes.

> > It just makes sense to point out, in the doc for these
> > two variables that, by default, `M-s SPC' toggles them
> > while searching.
> 
> Like I said, I disagree.
> 
> Maybe we should provide defcustoms that would initialize the values of
> these variables.  Then it would make sense to tell what you want in
> those defcustoms' doc strings, like we do with search-invisible, for
> example.

That takes care of letting users set the default behavior,
yes.

It doesn't, however, tell them anything about being able
to hit a key while searching to toggle the current behavior.

If you add defcustoms and then mention in their doc strings
that the _current_ behavior can be toggled with `M-s SPC'
then that would take care of this bug report.

Users will naturally want to (1) set their preferred default
behavior or (2) toggle the current behavior, or both.  The
point is to help them better to know how they can do these
things (which they can do already).