Clarify SE-0253: introducing callables. (#1014)
Clarify SE-0253: introducing callables. (#1014)

* Clarify SE-0253: introducing callables.

- Clearly state supported/unsupported modifiers/attributes for `call`
  declarations.
- Add an early blurb stating that alternative syntaxes are explored in the
  "Alternatives considered" section.
- Other minor changes.

* Minor rephrasing.

ddunbar/swift-evolution,ddunbar/swift-evolution,abertelrud/swift-evolution,natecook1000/swift-evolution,apple/swift-evolution,natecook1000/swift-evolution,Nirma/swift-evolution,apple/swift-evolution,hartbit/swift-evolution,anandabits/swift-evolution,DevAndArtist/swift-evolution,Nirma/swift-evolution,abertelrud/swift-evolution,hartbit/swift-evolution,anandabits/swift-evolution,DevAndArtist/swift-evolution