bug#5122: Mismatched parentheses when dealing with huge buffercontent

> > See option `blink-matching-paren-distance' and node 
> > Matching in the Emacs manual.
> > Does increasing that variable's value resolve the problem?
> 
> Yes, it does solve the problem.

It's not real obvious when you see mismatch highlighting (and are used to it
working) that the highlighting might not mean a mismatch at all, but just an
insufficiently large option value. Many (most?) users will not even guess that
there is such an option.

Perhaps we could use a :help-echo property along with the mismatch highlight
face, pointing to the option?