bug#21635: 24.5; dabbrev: customizable variable dabbrev-friend-buffer-function not documented in Emacs manual

[phs <phs@lsv.ens-cachan.fr>]

$ emacs -Q -l dabbrev --eval="(describe-variable 
'dabbrev-friend-buffer-function)"

Unless I am mistaken, there is no mention of 
dabbrev-friend-buffer-function in the
Emacs manual. This symbol, if bound, should contain a function that 
given a buffer
argument will tell if the buffer should be searched while 
dabbrev-expand'ing.

Further dabbrev-friend-buffer-function is initially bound to 
dabbrev--same-major-mode-p, with the effect that
only buffers with same major mode are searched through. This behaviour, 
while often welcome, can be treacherous at times,
and is not documented in the manual.

This may have a connection with bug #17899

In GNU Emacs 24.5.1 (x86_64-apple-darwin13.4.0, NS apple-appkit-1265.21)
  of 2015-04-10 on builder10-9.porkrind.org
Windowing system distributor `Apple', version 10.3.1404
Configured using:
  `configure --with-ns '--enable-locallisppath=/Library/Application
  Support/Emacs/${version}/site-lisp:/Library/Application 
Support/Emacs/site-lisp''

Important settings:
   value of $LC_ALL: en_US
   value of $LANG: en_US
   locale-coding-system: utf-8-unix

-- 
Philippe SCHNOEBELEN,       	                  www.lsv.ens-cachan.fr/~phs
LSV, CNRS UMR 8643 & ENS de Cachan   		  Tel: (+33/0) 147 407 530
"Algebraic symbols are used when you do not know what you are talking about"