Our attribute vs property resolution algorithm should be documented

[dead-claudia]

Currently, there's no mention in the relevant documentation about how we tell attributes and properties apart. It's pretty simple to explain: if prop in vnode.dom, it's treated as a property, and if not, it's treated as an attribute. This is a pretty simple thing to add, and it could probably be done within an hour if that.

We do need to mention that in the case of ill-behaved custom elements where identically-named properties and attributes exist, you should use oncreate/onupdate to update it if Mithril does the wrong thing (like if you need to update the attribute and not the property).

/cc @barneycarroll @porsager

[dead-claudia]

Forgot to closed - it's fixed now.