Camera: Document LEGACY not being allowed on new devices

Bug: 130315905
Test: Builds, docs look ok.
Change-Id: I9e1bd8561819f84419cac4d048085700b4694473
diff --git a/core/java/android/hardware/camera2/CameraMetadata.java b/core/java/android/hardware/camera2/CameraMetadata.java
index 9e0ee58..d3575c0 100644
--- a/core/java/android/hardware/camera2/CameraMetadata.java
+++ b/core/java/android/hardware/camera2/CameraMetadata.java
@@ -1252,6 +1252,7 @@
      * fire the flash for flash power metering during precapture, and then fire the flash
      * for the final capture, if a flash is available on the device and the AE mode is set to
      * enable the flash.</p>
+     * <p>Devices that initially shipped with Android version {@link android.os.Build.VERSION_CODES#Q Q} or newer will not include any LEGACY-level devices.</p>
      *
      * @see CaptureRequest#CONTROL_AE_PRECAPTURE_TRIGGER
      * @see CameraCharacteristics#REQUEST_AVAILABLE_CAPABILITIES