migration guide for combinator systems

Author: janis-bhm

release-content/migration-guides/combinator_system.md

@@ -0,0 +1,25 @@
+---
+title: System Combinators
+pull_requests: [20671]
+---
+
+The `CombinatorSystem`s can be used to combine multiple `SystemCondition`s with logical operators. Previously, the conditions would short circuit if the system failed to run, for example because it's query could not be filled by the world.
+Now, if a `SystemCondition` fails, it will be considered to have returned `false` and in combinators that don't short circuit the other condition will now be run.
+
+```rust
+fn vacant(_: crate::system::Single<&Vacant>) -> bool {
+    true
+}
+
+fn is_true() -> bool {
+    true
+}
+
+assert!(world.query::<&Vacant>().iter(&world).next().is_none());
+
+// previously:
+assert!(world.run_system_once(is_true.or(vacant)).is_err());
+
+// now:
+assert!(matches!(world.run_system_once(is_true.or(vacant)), Ok(true)));
+```