Add a missing implicit Copyable constraint

Co-authored-by: Kavon Farvardin <[email protected]>
Co-authored-by: Slava Pestov <[email protected]>
(cherry picked from commit 6fa34bd)

Author: 3 people

stdlib/public/core/Misc.swift

@@ -185,6 +185,7 @@ func _rethrowsViaClosure(_ fn: () throws -> ()) rethrows {
 /// * Protocol declarations
 /// * Associated type declarations
 /// * The `Self` type in a protocol extension
+/// * In an extension, the generic parameters of the type being extended
 ///
 /// A class or actor can contain noncopyable stored properties,
 /// while still being copyable itself ---