Clarify Language Version Selection in Visual Studio Documentation. (#43565)

* Fixed bug 43428.

* Apply suggestions from code review

---------

Co-authored-by: Adit Sheth <[email protected]>
Co-authored-by: Bill Wagner <[email protected]>

Author: 3 people

docs/csharp/language-reference/configure-language-version.md

@@ -22,6 +22,14 @@ If you must specify your C# version explicitly, you can do so in several ways:
 >
 > To know what language version you're currently using, put `#error version` (case sensitive) in your code. This makes the compiler report a compiler error, CS8304, with a message containing the compiler version being used and the current selected language version. See [#error (C# Reference)](preprocessor-directives.md#error-and-warning-information) for more information.
 
+## Why you can't select a different C# version in Visual Studio
+
+In Visual Studio, the option to change the language version through the UI might be disabled because the default version is aligned with the project's target framework (`TFM`). This default configuration ensures compatibility between language features and runtime support.
+
+For example, changing the target `TFM` (e.g., from [.NET 6](https://dotnet.microsoft.com/en-us/download/dotnet/6.0) to [.NET 9](https://dotnet.microsoft.com/en-us/download/dotnet/9.0)) will update the language version accordingly, from C# 10 to C# 13. This approach prevents issues with runtime compatibility and minimizes unexpected build errors due to unsupported language features.
+
+If you need a specific language version that differs from the one automatically selected, refer to the methods below to override the default settings directly in the project file.
+
 ## Edit the project file
 
 You can set the language version in your project file. For example, if you explicitly want access to preview features, add an element like this: