bug#3276: 23.0.93; doc for minibuffer-frame-alist

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)'
 
Doc string says:
 
 Alist of parameters for initial minibuffer frame.
 You can set this in your init file; for example,
 
  (setq minibuffer-frame-alist
        '((top . 1) (left . 1) (width . 80) (height . 2)))
 
 Parameters specified here supersede the values given in
 `default-frame-alist', for a minibuffer frame.
 
But this option is *only* for a standalone minibuffer frame, and such
a frame must have this frame parameter: (minibuffer . only). So that
example is a bad one.
 
Elisp manual says:
 
 This variable's value is an alist of parameter values used when
 creating an initial minibuffer-only frame--if such a frame is
 needed, according to the parameters for the main initial frame.
 
What does "if such a frame is needed" mean here?  When (why) would it
be needed?  What does "according to the parameters for the main
initial frame" mean here? 
 
This description is less informative than the doc string. And like the
doc string, it should mention that parameter `minibuffer' should have
value `only'.