Adds some extra comment/documentation for DebugMessenger in VkInstance.

Bug: chromium:1372772
Change-Id: I785ebf851d01ad06476be6b5f08ddf21e42f4116
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/70028
Commit-Queue: Loko Kung <lokokung@google.com>
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Tested-by: Loko Kung <lokokung@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
diff --git a/src/Vulkan/VkInstance.hpp b/src/Vulkan/VkInstance.hpp
index 129bde3..702547b 100644
--- a/src/Vulkan/VkInstance.hpp
+++ b/src/Vulkan/VkInstance.hpp
@@ -39,6 +39,9 @@
 
 private:
 	VkPhysicalDevice physicalDevice = VK_NULL_HANDLE;
+
+	// This debug messenger (when available) is owned by the instance and created via passing a
+	// chained struct to vkCreateInstance. This should be destroyed with the instance.
 	DebugUtilsMessenger *messenger = nullptr;
 };