Use an unique identifier for the different grammars documented using
the Sphinx productionlist markup.
productionlist markups of the same grammar, like "expressions" or
"compound statements", use the same identifier "python-grammar".
Document extended iterable unpacking, raising and catching exceptions, class decorators, etc.
A few notable things are not documented yet, I've added XXX comments about that.
Victor Stinner
Andrés Delfino
Georg Brandl
Ezio Melotti