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

[Gregory Heytings]

> I don't see any reason to delay landing the important improvements we 
> have on the branch.  It will allow more people to use those 
> improvements, and thus will contribute both to their stability and to 
> further progress in this direction.

Okay, I pushed two final (?) improvements: a new optional argument "lock" 
to narrow-to-region that Lars (added in Cc) suggested and which is now 
used to lock the restriction around fontification-functions, and two 
updates to the documentation.

Could you please tell me if it's ready to merge?

> > And we can/should open a new bug report and branch to discuss these 
> > long-and-truncated lines issues and solution attempts.
>
> Opening a new bug about that is fine by me.

Actually there's no need to open a new bug report, as bug#56683 already 
exists.