- clarify rationale for the extra TargetScopeError cases (we don't
  want current CPython implementation details to implicitly leak into
  the "do what CPython does" de facto language specification)
- broaden one of the error cases to handle the fact that CPython's
  symbol table analysis for a comprehension involves two different
  scopes and hence makes it difficult to detect when the target of
  a named expression in the iterable expression gets re-used as an
  iteration variable in the comprehension

- drop TargetScopeError from the PEP in favour of SyntaxError
- use the same marker for invalid constructs as the rest of the PEP
- clarify that the ban on named expressions in comprehension iterable
  expressions is consistent (rather than only being on the first one)
- provide a bit more detail on the CPython constraint that keeps us
  from implementing the "only complain on name conflicts" rule for
  iterable expressions
- add a new note on naming that covers both the informal name and
  the internal name in the reference implementation