node: clarify hacks for partially supported elements
node: clarify hacks for partially supported elements

I could not find conicalGradient and multiImage in the spec...
They are currently partially handled by aliasing them to
other elements (radialGradient and switch). These patch
just makes the hack more self-documenting, but does not
add real NodeType variants for these elements.

GNOME/librsvg,GNOME/librsvg,GNOME/librsvg,GNOME/librsvg