bug#33167: 26; Doc string of `region-extract-function'

[Drew Adams]

I don't understand these parts of the description:

 - nil: return the content as a string.

What does that mean, for a noncontiguous region?  Are the buffer
substrings for all of the region segments (zones) concatenated together?
If so, say that.

 - anything else: delete the region and return its content
   as a string, after filtering it with `filter-buffer-substring', which
   is called with METHOD as its 3rd argument.

So first the region is deleted, and the deleted content (see previous)
is filtered via `(filter-buffer-substring BEG END METHOD)' and then
returned.

Some of what I don't understand:

1. What are the BEG and END args passed to `filter-buffer-substring'?
   Is BEG the smallest car of any of the zones in the noncontiguous
   region, and END the largest cdr of any of the zones?

2. `filter-buffer-substring' calls the value of
   `filter-buffer-substring-function' with the same 3 args.  But what
   can that function do with BEG and END (which are what?)?  It's
   presumably a function that expects to use a single stretch of buffer
   text from BEG to END.  But here we're talking about a noncontiguous 

3. The 3rd arg to `filter-buffer-substring' just deletes the region from
   BEG to END if it is non-nil, so it seems like passing that non-nil
   3rd arg is useless, as the region gets deleted anyway, by
   `region-extract-function'.

4. The use of `filter-buffer-substring' is also unclear.  It is passed
   BEG and END (and METHOD, but see #3, above).  And it filters the
   buffer text between BEG and END.  But see #1 above - are BEG and END
   buffer positions that make sense for the whole region text?  Just
   what happens here?

This is quite unclear to me.  And following the rabbit hole from
`region-extract-function' down to `filter-buffer-substring' and then to
`filter-buffer-substring-function' does not make things more clear.

Or is what happens perhaps that EACH element of the noncontiguous
region, that is, each zone (BEG<N> . END<N>) of the list ((BEG1 . END1)
...) gets filtered by `filter-buffer-substring', passing its BEG END and
EMTHOD - so that a mapcar is applied?  In that case, how are the
resulting buffer substrings assembled - are they concatenated to get the
return value?

In GNU Emacs 26.1 (build 1, x86_64-w64-mingw32)
 of 2018-05-30
Repository revision: 07f8f9bc5a51f5aa94eb099f3e15fbe0c20ea1ea
Windowing system distributor `Microsoft Corp.', version 10.0.16299
Configured using:
 `configure --without-dbus --host=x86_64-w64-mingw32
 --without-compress-install 'CFLAGS=-O2 -static -g3''