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

[Drew Adams]

> > `C-h v isearch(-regexp)-lax-whitespace' tells you that command
> > `isearch-toggle-lax-whitespace' toggles the variable.  But it does not
> > tell you that this command is bound, by default, to `M-s SPC' during
> > Isearch.
> >
> > It should tell you that.  (It is not so easy to discover the binding.)
> 
> Those are variables, not defcustoms.  Why is it important to go to
> such lengths in their doc strings?

Go to such lengths?  Really?

See https://emacs.stackexchange.com/q/41960/105.

Sure, if you consult the doc of `isearch-forward(-regexp)'
or its keys, and you read it all, then when you get to line
43 (out of 82!) you'll find the info you're looking for wrt
lax-vs-strict whitespace matching:

 Type M-s SPC to toggle whitespace matching.
 In incremental searches, a space or spaces normally matches any 
 whitespace defined by the variable 'search-whitespace-regexp';
 see also the variables 'isearch-lax-whitespace' and
 'isearch-regexp-lax-whitespace'.

But if you don't do that then it is not so easy to pick
up the info about that key binding.

And even that text does not make clear what the latter
two vars do.  To discover how to change the default
behavior you need to actually click those var names
and consult their doc.

And yes, setting or binding those two vars really is
the way to go, to set your preferred default behavior.

We provide `search-whitespace-regexp' as a user option,
but not the other two variables, which are more commonly
set by users to govern whitespace matching for search.

Yes, we don't let those vars be options because of the
Emacs policy that code should not modify option values
(even the code for built-in toggles!).

It's an unfortunate policy, IMHO, but it is what it is.

It just makes sense to point out, in the doc for these
two variables that, by default, `M-s SPC' toggles them
while searching.

If providing that info to users is too much trouble
("go to such lengths"?), then ... please don't bother.