bug#9356: `event-modifiers' documentation

[Stefan Monnier]

> The documentation for `event-modifiers' states:
> "If EVENT is a symbol that has never been used in an event that has been
> read as input in the current Emacs session, then this function can return
> nil, even when EVENT actually has modifiers." However, looking in the
> event-modifier' definition, the function
> internal-event-symbol-parse-modifiers' seems to deal with this (potential)
> issue. Can someone give me an example where `event-modifier' would
> inappropriately return nil?

   (event-modifiers 'mouse-1) => (click)
   (event-modifiers 'mouse-10) => nil

Not sure if other modifiers than `click' can be missing.


        Stefan