typing docs: Clarify Protocol / runtime_checkable docs (#132606)

- Explicitly say that isinstance/issubclass do not work on non-runtime checkable
  protocols.
- Move the sentence "This raises TypeError when applied to a non-protocol class". It
  took me quite some time to decide what "this" was here: it refers to applying the
  decorator, not to an isinstance() call.

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>