Document ColorScheme usage for dark and light scheme selection (#4985)

* Document ColorScheme usage for theme selection

See vaadin/flow#22988 (comment)

* Add missing @

* Apply suggestion from @caalador

Co-authored-by: caalador <[email protected]>

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Co-authored-by: Jouni Koivuviita <[email protected]>
Co-authored-by: caalador <[email protected]>

Author: 3 people

articles/upgrading/index.adoc

@@ -260,6 +260,9 @@ The `themeFor` parameter of the [classname]`@CssImport` annotation (for shadow-D
 
 The special [filename]`document.css` file (for loading styles into the document root in embedded components) is removed as no longer necessary.
 
+You can use [classname]`@ColorScheme` for choosing between light or dark color scheme. 
+See <<{articles}/styling/themes#color-schemes, Color Schemes>> for more details on using color schemes.
+
 === Spring Security and StyleSheet
 
 By default, Vaadin Spring Security denies access to URLs that are not explicitly allowed in the <<{articles}/upgrading#security-configuration-changes, security configuration>>. To allow access to custom resources such as `@import 'css/foo.css'`, you need to configure permission for the relevant path in your security settings: