Re: tooltip-mode docstring

[Nick Roberts]

 >    A tooltip is, by definition, a window that pops up on mouse-over (it's
 >    called balloon help in the manual).
 > 
 > This assumes that the reader knows this.

I think it's reasonable to start with certain assumptions.  It's a doc string,
not a primer (or legal document).  tool-bar-mode doesn't explain what a
tool bar is, menu-bar-mode what a menu-bar is...

Nick