bug#13437: 24.3.50; doc of `but-last'

[Drew Adams]

Doc string:
 
 Return a copy of LIST with the last N elements removed.
 
It is better for the first line to describe the behavior with null
optional args.  E.g.:
 
 Return a copy of LIST with the last element removed.
 Optional arg N is the number of elements to remove.
 
Similarly, the doc string of `nbutlast' - e.g.,
 
 Modify LIST by removing the last element.
 Optional arg N is the number of elements to remove.

In GNU Emacs 24.3.50.1 (i386-mingw-nt5.1.2600)
 of 2012-12-31 on ODIEONE
Bzr revision: 111388 rudalics@gmx.at-20121231113513-subz2dazg6yjukzh
Windowing system distributor `Microsoft Corp.', version 5.1.2600
Configured using:
 `configure --with-gcc (4.7) --no-opt --enable-checking --cflags
 -IC:/Devel/emacs/build/include --ldflags -LC:/Devel/emacs/build/lib'