Improve -preview=safer changelog entry (#17490)

changelog/dmd.safer.dd

@@ -1,3 +1,22 @@
-This adds `-preview=safer` which enables safety checking on un-attributed functions.
+Add `-preview=safer` switch for safety checking on unattributed functions
+
+All the checks currently enabled in `@safe` code, that are easily fixed (as in
+the fix is constrained to the function), will be enabled in `-preview=safer` code.
+
+Code not easily fixed, such as calls to `@system` or unattributed functions, will
+be allowed as before.
+
+---
+void f();
+@system void g();
+
+void main()
+{
+    int* p;
+    p++; // Error, pointer arithmetic
+    f(); // allowed
+    g(); // allowed
+}
+---
 
 For more information, see: [safer.md](https://github.com/WalterBright/documents/blob/38f0a846726b571f8108f6e63e5e217b91421c86/safer.md)