Clarifies that cancelling a Task awaiting another Task or Future will
also cancel the awaited object. This behavior was undocumented despite
being fundamental to asyncio's cancellation architecture.

Changes:
- Updated general Task description to mention Task cancellation propagation
- Added explicit explanation in Task.cancel() method documentation
- Clarified that Tasks inherit Future's cancellation behavior

Addresses issue python#141186 where users were unaware this cancellation
propagation was intentional architectural behavior, not a side effect.

The fix uses the exact wording suggested by the issue reporter and
documents the _fut_waiter implementation behavior that enables the
propagation down entire await chains.