Please add a note about 12 / 16 byte IV compatibility

[Neustradamus]

Please add a note about 12 / 16 byte IV compatibility

Linked to:

• add defined IV size for GCM. clarified text xsf/xeps#894

[Echolon]

Hello, I think there are also many other and will be more issues about OMEMO. Yes this is a serious one. I think we should track on all OMEMO compatibilites. Let's keep it simple for the moment.
I would rather suggest to have an info box at the page top to inform about recent issues. I think that is not the pages purpose.
PR is welcome.