bug#3279: 23.0.93; doc for frame parameter `minibuffer'

The Elisp manual should say that you cannot change the value of
parameter `minibuffer' for an existing frame.
 
If you try to set it to nil, you get one error. If you try to set it
to a minibuffer window on another frame, you get a different error. If
you try to set it to `only', you get no error, but nothing happens.
We should say that this parameter is only for use in a frame alist
before the frame is created.
 

In GNU Emacs 23.0.93.1 (i386-mingw-nt5.1.2600)
 of 2009-05-02 on SOFT-MJASON
Windowing system distributor `Microsoft Corp.', version 5.1.2600
configured using `configure --with-gcc (3.4)'