bug#33179: 26; Doc string of `filter-buffer-substring'

[Drew Adams]

IIUC, the meaning of "extract" in this sentence is to exclude the chars
from the result.  If so, please consider using "exclude" or "remove"
(preferably the former, as nothing is removed from the buffer) instead.
"Extract" can give the impression that those chars are extracted as the
result, rather than being excluded from the result.

  "For example, major or minor modes can use
   'filter-buffer-substring-function' to extract characters that are
                                         ^^^^^^^
   special to a buffer, and should not be copied into other buffers."

Yes, if read carefully one can guess what was really meant here, because
of the "and should not..." part.  Still, the text can be misleading as
it is now.

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''