Remove a limitation on @objc protocol adoption.

For example, this works even though the C class isn't marked @objc,
isn't a subclass of NSObject, and isn't a subclass of an @objc class:

    import Foundation
    @objc protocol P {}
    class C: P {}

Co-authored-by: Slava Pestov <spestov@apple.com>

Author: amartini51

Sources/TSPL/TSPL.docc/LanguageGuide/Protocols.md

@@ -2145,9 +2145,8 @@ Optional requirements are available so that you can write code
 that interoperates with Objective-C.
 Both the protocol and the optional requirement
 must be marked with the `@objc` attribute.
-Note that `@objc` protocols can be adopted only by classes
-that inherit from Objective-C classes or other `@objc` classes.
-They can't be adopted by structures or enumerations.
+Note that `@objc` protocols can be adopted only by classes,
+not by structures or enumerations.
 
 When you use a method or property in an optional requirement,
 its type automatically becomes an optional.