Special hook run just before proceeding to fontify a region.
This is used to allow major modes to help font-lock find safe buffer positions
as beginning and end of the fontified region. Its most common use is to solve
the problem of /identification/ of multiline elements by providing a function
that tries to find such elements and move the boundaries such that they do
not fall in the middle of one.
Each function is called with no argument; it is expected to adjust the
dynamically bound variables `font-lock-beg' and `font-lock-end'; and return
non-nil if it did make such an adjustment.
These functions are run in turn repeatedly until they all return nil.
Put first the functions more likely to cause a change and cheaper to compute.