Commit 005143a4 completely broke all the
introspected plug-ins, so I am reverting only the relevant changes.

If some things need to be improved in this part of the docs, let's do it
without breaking major stuff.
In particular: abstract methods must be properly documented (with
parameters, return values and annotations) like any function, and must
be named Class::method, e.g. GimpPlugInClass::query_procedures.
Other we break GObject Introspection, i.e. we break all plug-ins using
these.