[Docs] Conditional conformances are implemented. (#26359)

Update Generics Manifesto to mark conditional conformances implemented.

Author: rxwei

docs/GenericsManifesto.md

@@ -283,7 +283,7 @@ Unlike the minor extensions, major extensions to the generics model provide more
 
 ### Conditional conformances (*)
 
-*This feature has been accepted in [SE-0143](https://github.com/apple/swift-evolution/blob/master/proposals/0143-conditional-conformances.md) and is under development.*
+*This feature has been accepted in [SE-0143](https://github.com/apple/swift-evolution/blob/master/proposals/0143-conditional-conformances.md) and is implemented in Swift 4.2.*
 
 Conditional conformances express the notion that a generic type will conform to a particular protocol only under certain circumstances. For example, `Array` is `Equatable` only when its elements are `Equatable`: