Improve the comments of Thermal HAL 2.0 API

Make sure (un)register API failed with nullptr

Test: Build
Test: VtsHalThermalV2_0TargetTest
Change-Id: I42a87fde85a38f77faa4d07f29ed3a78501a0dca
diff --git a/thermal/2.0/vts/functional/VtsHalThermalV2_0TargetTest.cpp b/thermal/2.0/vts/functional/VtsHalThermalV2_0TargetTest.cpp
index 3893014..cc2f3df 100644
--- a/thermal/2.0/vts/functional/VtsHalThermalV2_0TargetTest.cpp
+++ b/thermal/2.0/vts/functional/VtsHalThermalV2_0TargetTest.cpp
@@ -132,8 +132,13 @@
 TEST_F(ThermalHidlTest, RegisterThermalChangedCallbackTest) {
     // Expect to fail with same callback
     auto ret = mThermal->registerThermalChangedCallback(
-        mThermalCallback, false, TemperatureType::SKIN,
-        [](ThermalStatus status) { EXPECT_NE(ThermalStatusCode::SUCCESS, status.code); });
+            mThermalCallback, false, TemperatureType::SKIN,
+            [](ThermalStatus status) { EXPECT_EQ(ThermalStatusCode::FAILURE, status.code); });
+    ASSERT_TRUE(ret.isOk());
+    // Expect to fail with null callback
+    ret = mThermal->registerThermalChangedCallback(
+            nullptr, false, TemperatureType::SKIN,
+            [](ThermalStatus status) { EXPECT_EQ(ThermalStatusCode::FAILURE, status.code); });
     ASSERT_TRUE(ret.isOk());
     sp<ThermalCallback> localThermalCallback = new (std::nothrow) ThermalCallback();
     // Expect to succeed with different callback
@@ -141,11 +146,16 @@
         localThermalCallback, false, TemperatureType::SKIN,
         [](ThermalStatus status) { EXPECT_EQ(ThermalStatusCode::SUCCESS, status.code); });
     ASSERT_TRUE(ret.isOk());
-    // Remove the local callback.
+    // Remove the local callback
     ret = mThermal->unregisterThermalChangedCallback(
         localThermalCallback,
         [](ThermalStatus status) { EXPECT_EQ(ThermalStatusCode::SUCCESS, status.code); });
     ASSERT_TRUE(ret.isOk());
+    // Expect to fail with null callback
+    ret = mThermal->unregisterThermalChangedCallback(nullptr, [](ThermalStatus status) {
+        EXPECT_EQ(ThermalStatusCode::FAILURE, status.code);
+    });
+    ASSERT_TRUE(ret.isOk());
 }
 
 // Test Thermal->unregisterThermalChangedCallback.