Note source for ivars being guaranteed to be zero-initialized

crates/objc2/src/declare/ivar.rs

@@ -30,12 +30,12 @@ pub(crate) mod private {
 //
 // Additionally, the type must be safe to drop even if zero-initialized.
 //
-// Note that while I couldn't find a reference on whether ivars are
-// zero-initialized or not, it has been true since the Objective-C version
-// shipped with Mac OS X 10.0 [link] and is generally what one would expect
-// coming from C. So I think it's a valid assumption to make!
+// Ivars are documented to be zero-initialized in [this section of the
+// Objective-C manual][obj-dynamically-created], and that has been true since
+// at least [the Objective-C version shipped with Mac OS X 10.0][objc4-208].
 //
-// [link]: https://github.com/apple-oss-distributions/objc4/blob/objc4-208/runtime/objc-class.m#L367
+// [obj-dynamically-created]: https://developer.apple.com/library/archive/documentation/Cocoa/Conceptual/ProgrammingWithObjectiveC/WorkingwithObjects/WorkingwithObjects.html#//apple_ref/doc/uid/TP40011210-CH4-SW7
+// [objc4-208]: https://github.com/apple-oss-distributions/objc4/blob/objc4-208/runtime/objc-class.m#L367
 pub unsafe trait InnerIvarType: private::Sealed + Encode {
     /// The type that an `Ivar` containing this will dereference to.
     ///