[Stdlib] Reconstitute the documentation part of the comment on AnyKeyPath.

Author: mikeash

stdlib/public/core/KeyPath.swift

@@ -26,12 +26,14 @@ internal func _abstract(
 
 // MARK: Type-erased abstract base classes
 
-// A type-erased key path, from any root type to any resulting value
-// type. NOTE: older runtimes had Swift.AnyKeyPath as the ObjC name.
+// NOTE: older runtimes had Swift.AnyKeyPath as the ObjC name.
 // The two must coexist, so it was renamed. The old name must not be
 // used in the new runtime. _TtCs11_AnyKeyPath is the mangled name for
 // Swift._AnyKeyPath.
 @_objcRuntimeName(_TtCs11_AnyKeyPath)
+
+/// A type-erased key path, from any root type to any resulting value
+/// type.
 public class AnyKeyPath: Hashable, _AppendKeyPath {
   /// The root type for this key path.
   @inlinable