Update the draft of the `@backDeploy` proposal to address feeback in the forum pitch thread:
Update the draft of the `@backDeploy` proposal to address feeback in the forum pitch thread:

- Make better arguments against the alternative design in which the `@available` attribute is augmented instead of introducing `@backDeploy`.
- Reference "polyfill" as an existing term of art.
- Add a bit more detail to the Future directions section.

apple/swift-evolution,apple/swift-evolution