bug#56682: Fix the long lines font locking related slowdowns

[Gregory Heytings]

> Please always assume that I gave these aspects and your perspective due 
> consideration before making my changes, and please never revert them 
> without discussing first.

Okay, I'll keep that in mind in the future.

> In this case, "arbitrarily large" contradicts the text that follows, 
> which describes the circumstances where that might not be true.

I've carefully chosen the words of the title, and it doesn't contradict 
what follows, as far as I understand.  It says "Emacs is now capable of 
editing files with arbitrarily long lines", in which "capable" means that 
it can do it, but will not always do it.  The circumstances that are 
described in the text that follows tell the reader that the remaining 
cases in which Emacs would choke on such files are outside of Emacs' 
responsibility, they are the responsibility of major and minor mode 
writers.