bug#20232: 25.0.50; doc string of `multi-isearch-buffer-list'

[Drew Adams <drew.adams@oracle.com>]

Please add a doc string.  Seems that if anything in `misearch.el'
deserves a doc string it is this variable.

This is presumably the easiest and best thing for code to test, to see
if Isearch is currently searching more than one buffer.  At least a
glance at the code gives me that impression.  It seems better, I'm
guessing, than testing either of these:

* (and multi-isearch-next-buffer-current-function
       (buffer-live-p multi-isearch-current-buffer))

* Option multi-isearch-search.


In GNU Emacs 25.0.50.1 (i686-pc-mingw32)
 of 2014-10-20 on LEG570
Bzr revision: 118168 rgm@gnu.org-20141020195941-icp42t8ttcnud09g
Windowing system distributor `Microsoft Corp.', version 6.1.7601
Configured using:
 `configure --enable-checking=yes,glyphs CPPFLAGS=-DGLYPH_DEBUG=1'