This variable is a wrapper hook around `filter-buffer-substring'.
Each member of the hook should be a function accepting four arguments:
(FUN BEG END DELETE), where FUN is itself a function of three arguments
(BEG END DELETE). The arguments BEG, END, and DELETE are the same
as those of `filter-buffer-substring' in each case.
The first hook function to be called receives a FUN equivalent
to the default operation of `filter-buffer-substring',
i.e. one that returns the buffer-substring between BEG and
END (processed by any `buffer-substring-filters'). Normally,
the hook function will call FUN and then do its own processing
of the result. The next hook function receives a FUN equivalent
to the previous hook function, calls it, and does its own
processing, and so on. The overall result is that of all hook
functions acting in sequence.
Any hook may choose not to call FUN though, in which case it
effectively replaces the default behavior with whatever it chooses. Of course, a later hook function may do the same thing.