bug#22023: 25.0.50; doc string of `eval-buffer'

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

The doc string suggests that the entire buffer is always evaluated,
which is wrong.

The Elisp manual, node `Eval' is better.  It says that it operates on
the entire _accessible portion_ of the buffer.  That's a big difference.

This function is commonly used by users, as a command.  They should
realize that they can easily limit its scope by narrowing the buffer.

Even the treatment in the manual could be improved by making clear what
is meant by the "accessible" portion of the buffer.  When the Emacs
manual refers to the "accessible portion" it follows that phrase with
this: (*note Narrowing::).  That makes everything clear.

In GNU Emacs 25.0.50.1 (i686-pc-mingw32)
 of 2015-11-12
Bzr revision: 6e5186e8a7ccfb9b8fb35b5f4f0371e4f4a68162
Windowing system distributor `Microsoft Corp.', version 6.1.7601
Configured using:
 `configure --prefix=/c/Devel/emacs/snapshot/trunk
 --enable-checking=yes,glyphs --enable-check-lisp-object-type
 --without-compress-install --with-wide-int 'CFLAGS=-O0 -ggdb3'
 LDFLAGS=-Lc:/Devel/emacs/lib 'CPPFLAGS=-DGC_MCHECK=1
 -Ic:/Devel/emacs/include''