Handling of XML namespaces

Right now, Svg and Html functors takes the same XML parameter, but their respective elements should not be instantiated in the same namespace (svg elements "live" in http://www.w3.org/2000/svg).
The current workaround is to pass a different XML instance, manually specialized for a namespace.

But this is also done explicitly by Tyxml only for the root svg element: https://github.com/ocsigen/tyxml/blob/master/lib/svg_f.ml#L740-L742. It would be nice if the behavior was consistent: pass the namespace for all elements or for none.
Actually, none is probably a better solution since, as far as the DOM is concerned, xmlns is not handled like a normal attribute (it changes the meaning of the rest of the syntax).

It seems ok if each instance of Xml has to handle that internally. This caveat should just be properly documented.

Start by reading the namespace handling around lib/svg_f.ml#L740-L742 and the Svg and Html functor interfaces. Compare how a shared XML instance is used and how the root SVG element is treated. Done means the namespace behavior is decided consistently and the resulting caveat is documented.