bug#17263: 24.4.50; doc string of `backward-delete-char-untabify': active-region behavior

[Drew Adams]

> > The doc string should, like that of `backward-delete-char' (aka
> > `delete-backward-char'), mention the behavior with `transient-mark-mode'
> > and its dependence on `delete-active-region'.
> 
> Do you have a suggestion for a doc string?  I don't know what that
> command does when there's an active region.

I think it is the same as for `backward-delete-char' wrt the
deletion part.  This is the text from `backward-delete-char'
regarding the region:

 If Transient Mark mode is enabled, the mark is active, and N is 1,
 delete the text in the region and deactivate the mark instead.
 To disable this, set option `delete-active-region' to nil.