Add CHANGELOG.md entry describing previous rethrows checking change and subsequent relaxation

Author: slavapestov

CHANGELOG.md

@@ -29,6 +29,24 @@ CHANGELOG
 Swift 5.5
 ---------
 
+* The determination of whether a call to a `rethrows` function can throw now considers default arguments of `Optional` type.
+
+  In Swift 5.4, such default arguments were ignored entirely by `rethrows` checking. This meant that the following example was accepted:
+
+  ```swift
+  func foo(_: (() throws -> ())? = nil) rethrows {}
+  foo()  // no 'try' needed
+  ```
+
+  However, it also meant that the following was accepted, even though the call to `foo()` can throw and the call site is not marked with `try`:
+
+  ```swift
+  func foo(_: (() throws -> ())? = { throw myError }) rethrows {}
+  foo()  // 'try' *should* be required here
+  ```
+
+  The new behavior is that the first example is accepted because the default argument is syntactically written as `nil`, which is known not to throw. The second example is correctly rejected, on account of missing a `try` since the default argument *can* throw.
+
 * [SE-0293][]:
 
   Property wrappers can now be applied to function and closure parameters: