Start documenting the toplevel rsvg_internals/lib.rs
Start documenting the toplevel rsvg_internals/lib.rs

"cargo doc --document-private-items" actually produces pretty nice
output, if we care to document the internals.  Hopefully this should
make the library sources easier to explore.

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