Improve documentation for QML Shortcut's sequence property

When using a standard key for Shortcut's sequence property, I found that if the key resolves to multiple sequences, we get this warning and only the first sequence gets assigned to the shortcut:

QML Shortcut: Shortcut: Only binding to one of multiple key bindings associated with 15. Use 'sequences: [ <key> ]' to bind to all of them.

And as the message says, we have to use the sequences property instead, assigning it a single-element array.

As this can result in very nasty bugs, I think the documentation should say something about it. Maybe even emphasize that using sequence is not a reliable way to assign standard keys to shortcuts (you may not know if, depending on the platform, it will resolve to one or multiple sequences) and that sequences should be preferred in this case.