bug#5122: Mismatched parentheses when dealing with hugebuffercontent

[Drew Adams]

> If you are really worried about the intrusiveness of an 
> explanation in the echo message or in a tooltip (remember,
> the mouse would need to be over the "mismatched" paren
> for the tooltip message to appear, and the message would
> only need to be displayed when the threshold is in fact 
> surpassed), then consider simply _not highlighting_ at all
> whenever the search distance has been exceeded.

Another suggestion - when the limit is reached:

1. Don't highlight this "mismatch".

2. Display message stating that we hit the limit defined by the option.

3. Open the help buffer for the option.

#3 is the new part of the suggestion. It lets users get complete info about the
option, and lets them click to go to Customize to change the value (e.g.
temporarily).

Otherwise, they need to open *Messages*, get the variable name, and then use
`set-variable' or `M-x customize-option', to change the value.