Add a note to the branding documentation to state setting appname is only applicable for basic theme.

docs/en/UI/AspNetCore/Branding.md

@@ -26,6 +26,8 @@ namespace MyProject.Web
 }
 ````
 
+> Currently, setting the `AppName` is only applicable to the [Basic Theme](../../Themes/Basic.md), it does not have any effect on the other [official themes](../../Themes/Index.md).
+
 The result will be like shown below:
 
 ![bookstore-added-logo](../../images/bookstore-added-logo.png)

docs/en/UI/Blazor/Branding.md

@@ -24,6 +24,8 @@ namespace MyCompanyName.MyProjectName.Blazor
 }
 ````
 
+> Currently, setting the `AppName` is only applicable to the [Basic Theme](../../Themes/Basic.md), it does not have any effect on the other [official themes](../../Themes/Index.md).
+
 The result will be like shown below:
 
 ![branding-appname](../../images/branding-appname.png)