bug#4227: 23.1; doc string of use-file-dialog

[Drew Adams]

>From the doc string:
 
 The value of `use-dialog-box' takes precedence over this
 variable, so a file dialog is only used if
 both `use-dialog-box' and this variable are non-nil.
 
The first part of this sentence is wrong, and is contradicted by the
second part. The default value of the option is t. To turn off using
the file dialog box it is sufficient to set the option to nil -
regardless of the value of `use-dialog-box'. To turn it on, yes, you
need to have both set to non-nil - but that still does not mean that
"`use-dialog-box' takes precedence".
 
Just remove the first part of that sentence.
 

In GNU Emacs 23.1.1 (i386-mingw-nt5.1.2600)
 of 2009-07-29 on SOFT-MJASON
Windowing system distributor `Microsoft Corp.', version 5.1.2600
configured using `configure --with-gcc (4.4)'