Document null-probing semantic differences in singleton migrations (#1081)

`List.of(..)`, `Set.of(..)`, and `Map.of(..)` throw `NullPointerException`
when probed with `contains(null)` and friends, whereas the
`Collections.singleton*` factories return `false`/`-1`/`null`. Call this
out in each recipe description so users can decide whether the migration
is safe for their code. Also fixes a `Set.Of` typo.

Author: timtebeek

src/main/java/org/openrewrite/java/migrate/util/MigrateCollectionsSingletonList.java

@@ -38,7 +38,9 @@ public class MigrateCollectionsSingletonList extends Recipe {
     final String displayName = "Prefer `List.of(..)`";
 
     @Getter
-    final String description = "Prefer `List.of(..)` instead of using `Collections.singletonList()` in Java 9 or higher.";
+    final String description = "Prefer `List.of(..)` instead of using `Collections.singletonList()` in Java 9 or higher. " +
+            "Note that the resulting `List` is not behaviorally equivalent: `List.of(..)` throws `NullPointerException` when probed with `contains(null)`, `indexOf(null)`, or `lastIndexOf(null)`, " +
+            "whereas `Collections.singletonList(..)` returns `false`/`-1`.";
 
     @Override
     public TreeVisitor<?, ExecutionContext> getVisitor() {

src/main/java/org/openrewrite/java/migrate/util/MigrateCollectionsSingletonMap.java

@@ -40,7 +40,9 @@ public class MigrateCollectionsSingletonMap extends Recipe {
     final String displayName = "Prefer `Map.of(..)`";
 
     @Getter
-    final String description = "Prefer `Map.of(..)` instead of using `Collections.singletonMap()` in Java 9 or higher.";
+    final String description = "Prefer `Map.of(..)` instead of using `Collections.singletonMap()` in Java 9 or higher. " +
+            "Note that the resulting `Map` is not behaviorally equivalent: `Map.of(..)` throws `NullPointerException` when probed with `containsKey(null)`, `containsValue(null)`, or `get(null)`, " +
+            "whereas `Collections.singletonMap(..)` returns `false`/`null`.";
 
     @Override
     public TreeVisitor<?, ExecutionContext> getVisitor() {

src/main/java/org/openrewrite/java/migrate/util/MigrateCollectionsSingletonSet.java

@@ -36,7 +36,9 @@ public class MigrateCollectionsSingletonSet extends Recipe {
     final String displayName = "Prefer `Set.of(..)`";
 
     @Getter
-    final String description = "Prefer `Set.Of(..)` instead of using `Collections.singleton()` in Java 9 or higher.";
+    final String description = "Prefer `Set.of(..)` instead of using `Collections.singleton()` in Java 9 or higher. " +
+            "Note that the resulting `Set` is not behaviorally equivalent: `Set.of(..)` throws `NullPointerException` when probed with `contains(null)`, " +
+            "whereas `Collections.singleton(..)` returns `false`.";
 
     @Override
     public TreeVisitor<?, ExecutionContext> getVisitor() {