[vulkan] Instruct users to install vulkan sdk if they want to use val…

…idation layer (#6098)

Issue:  #2713 

### Brief Summary

taichi/rhi/vulkan/vulkan_device_creator.cpp

@@ -267,7 +267,10 @@ void VulkanDeviceCreator::create_instance(bool manual_create) {
 
   if (params_.enable_validation_layer) {
     if (!check_validation_layer_support()) {
-      TI_WARN("validation layers requested but not available, turning off...");
+      TI_WARN(
+          "Validation layers requested but not available, turning off... "
+          "Please make sure Vulkan SDK from https://vulkan.lunarg.com/sdk/home "
+          "is installed.");
       params_.enable_validation_layer = false;
     }
   }
@@ -541,6 +544,8 @@ void VulkanDeviceCreator::create_logical_device(bool manual_create) {
     } else if (name == VK_KHR_BUFFER_DEVICE_ADDRESS_EXTENSION_NAME) {
       enabled_extensions.push_back(ext.extensionName);
     } else if (name == VK_KHR_SHADER_NON_SEMANTIC_INFO_EXTENSION_NAME) {
+      // VK_KHR_shader_non_semantic_info isn't supported on molten-vk.
+      // Tracking issue: https://github.com/KhronosGroup/MoltenVK/issues/1214
       ti_device_->set_cap(DeviceCapability::spirv_has_non_semantic_info, true);
       enabled_extensions.push_back(ext.extensionName);
     } else if (std::find(params_.additional_device_extensions.begin(),