Better document need for declare_class! when conforming to protocols

crates/objc2/src/macros/extern_protocol.rs

@@ -7,9 +7,12 @@
 /// general information about protocols in Objective-C.
 ///
 /// This macro will create an `unsafe` trait with methods which all have
-/// default implementations, such that an object that conforms to the protocol
-/// can write `unsafe impl MyProtocol for MyClass {}`, and get access to the
-/// functionality exposed by the protocol.
+/// default implementations, such that an external class that conforms to the
+/// protocol can write `unsafe impl MyProtocol for MyClass {}`, and get access
+/// to the functionality exposed by the protocol.
+///
+/// Note that that conforming to a protocol in a custom object requires
+/// putting the implementation inside the [`declare_class!`] invocation.
 ///
 /// Objective-C has a smart feature where you can write `id<MyProtocol>`, and
 /// then work with the protocol as-if it was an object; this is very similar