IFace: Add more comments

Bug: 152412683
Test: N/A
Change-Id: I0d444804acdd7e063c411cc2dec05efb26c6facf

biometrics/face/aidl/android/hardware/biometrics/face/EnrollmentType.aidl

@@ -19,6 +19,15 @@ package android.hardware.biometrics.face;
 @VintfStability
 @Backing(type="byte")
 enum EnrollmentType {
+    /**
+     * Default enrollment type.
+     */
     DEFAULT,
+
+    /**
+     * Enrollment type for people with limited vision or mobility. For example,
+     * enrollment of this type will not ask the user to move their head or
+     * look directly at the device.
+     */
     ACCESSIBILITY,
 }

biometrics/face/aidl/android/hardware/biometrics/face/FaceSensorType.aidl

@@ -16,4 +16,23 @@
 
 package android.hardware.biometrics.face;
 
-@VintfStability @Backing(type="byte") enum FaceSensorType { UNKNOWN, RGB, IR }
+@VintfStability
+@Backing(type="byte")
+enum FaceSensorType {
+    /**
+     * Placeholder value used for default initialization of FaceSensorType.
+     * This value means FaceSensorType wasn't explicitly initialized and must
+     * be discarded by the recipient.
+     */
+    UNKNOWN,
+
+    /**
+     * The face sensor is an RGB camera.
+     */
+    RGB,
+
+    /**
+     * The face sensor is an infrared camera.
+     */
+    IR,
+}