Update SE-0364 to new @retroactive syntax

[harlanhaskins]

This updates the proposal to the new @retroactive Protocol syntax and expands on the attribute's semantics.

[benrimmington]

In the "proposed solution" section, please could you:

• split the warning across several lines (to avoid scrolling);
• enable syntax highlighting for the second example.

Should the imported type and protocol be fully-qualified in the warning message?

// warning: extension declares a conformance of imported type 'Foundation.Date'
// to imported protocol 'Swift.Identifiable'; this will not behave correctly if
// the owners of 'Foundation' introduce this conformance in the future
extension Date: Identifiable {
^