docs: Update gi-docgen annotations
The whole `SECTION` block is a gtk-doc specific annotation. Just immediately document the class/struct/interface with the usual docs syntax and gi-docgen will know what to do with it. There's also no worrying about exposed structs, since there are none in libshumate, and even if there were, you would still have to document them in the GIR. Finally, use proper linking as specified by the gi-docgen tutorial.