[Docs] Extensions - better wording for c# equivalent (shader-slang#5636)

slang extensions look pretty same c# extension methods, not partial classes. Partial classes allow extending types to introduce more variables unlike slang extensions. Also c# users that use extension methods much more  than partial types, probably.

Co-authored-by: Yong He <yonghe@outlook.com>

Author: burak-efe

docs/user-guide/03-convenience-features.md

@@ -567,7 +567,7 @@ void test()
 }
 ```
 
-This feature is similar to extensions in Swift and partial classes in C#.
+This feature is similar to extensions in Swift and extension methods in C#.
 
 > #### Note:
 > You can only extend a type with additional methods. Extending with additional data fields is not allowed.