Add an explanation for PatternMatchingInstanceof

PiperOrigin-RevId: 736288615

Author: cushon

docs/bugpattern/PatternMatchingInstanceof.md

@@ -0,0 +1,28 @@
+Pattern matching with `instanceof` allows writing this:
+
+```java
+void handle(Object o) {
+  if (o instanceof Point(int x, int y)) {
+    handlePoint(x, y);
+  } else if (o instanceof String s) {
+    handleString(s);
+  }
+}
+```
+
+which is more concise than an instanceof and a separate cast:
+
+```java
+void handle(Object o) {
+  if (o instanceof Point) {
+    Point point = (Point) o;
+    handlePoint(point.x(), point.y());
+  } else if (o instanceof String) {
+    String s = (String) o;
+    handleString(s);
+  }
+}
+```
+
+For more information on pattern matching and `instanceof`, see
+[Pattern Matching for the instanceof Operator](https://docs.oracle.com/en/java/javase/21/language/pattern-matching-instanceof.html)