Clarified introduction about the abstract keyword (#12232)

Author: svick

docs/csharp/language-reference/keywords/abstract.md

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 # abstract (C# Reference)
-The `abstract` modifier indicates that the thing being modified has a missing or incomplete implementation. The abstract modifier can be used with classes, methods, properties, indexers, and events. Use the `abstract` modifier in a class declaration to indicate that a class is intended only to be a base class of other classes. Members marked as abstract, or included in an abstract class, must be implemented by classes that derive from the abstract class.  
+The `abstract` modifier indicates that the thing being modified has a missing or incomplete implementation. The abstract modifier can be used with classes, methods, properties, indexers, and events. Use the `abstract` modifier in a class declaration to indicate that a class is intended only to be a base class of other classes, not instantiated on its own. Members marked as abstract must be implemented by classes that derive from the abstract class.
 
 ## Example  
  In this example, the class `Square` must provide an implementation of `Area` because it derives from `ShapesClass`: