Update CHANGELOG for a recent source-breaking fix to 'findProtocolSelfReferences' (#39636)

Author: AnthonyLatsis

CHANGELOG.md

@@ -6,6 +6,28 @@ _**Note:** This is in reverse chronological order, so newer entries are added to
 Swift 5.6
 ---------
 
+* References to `Self` or so-called "`Self` requirements" in the type signatures
+  of protocol members are now correctly detected in the parent of a nested type.
+  As a result, protocol members that fall under this overlooked case are no longer
+  available on values of protocol type:
+
+  ```swift
+  struct Outer<T> {
+    struct Inner {}
+  }
+
+  protocol P {}
+  extension P {
+    func method(arg: Outer<Self>.Inner) {}
+  }
+
+  func test(p: P) {
+    // error: 'method' has a 'Self' requirement and cannot be used on a value of
+    // protocol type (use a generic constraint instead).
+    _ = p.method
+  }
+  ``` 
+
 * [SE-0324][]:
 
   Relax diagnostics for pointer arguments to C functions. The Swift