Revert "docs: temporarily disable the kernel-doc extension"

This reverts commit fd68a72875cf318f4310726f842139119c5f45d5.  We're
done with the update of kernel-doc and we can restore kernel-doc's
functionality.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

docs/sphinx/kerneldoc.py

@@ -80,9 +80,6 @@ def run(self):
         # Tell sphinx of the dependency
         env.note_dependency(os.path.abspath(filename))
 
-        # Disabled temporarily while scripts/kernel-doc is updated
-        return []
-
         tab_width = self.options.get('tab-width', self.state.document.settings.tab_width)
 
         # FIXME: make this nicer and more robust against errors