Complementary to GObjectIntrospection, we could do something like "Documentation Introspection" to collect library documentation (currently generated using gtk-doc) in one place. Language bindings could then generate appropriate documentation based on that using their own tools.
Documentation could either be stored with metadata in .gir files in an own xml namespace or separately.
GObjectIntrospection/Doctools is implementing this idea.
Another example (old)
Using existing tools
GTK-Doc is currently used to build documentation for libraries. From that, we could import documentation into .gir files with a new gtk-doc module (could be named gtkdoc-mkgir).
Valadoc could be used to write documentation metadata for Vala libraries to .gir files or to xml files along those.
Projects that could make use of Documentation Metadata
Vala - Compiler for the GObject type system
PyGObject - Python bindings
JGIR - Java/JVM bindings