Additional explanation for CS0501 (dotnet#29805)

* Update cs0501.md

* Update cs0501.md

* Update docs/csharp/misc/cs0501.md

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

Author: SeanKilleen

docs/csharp/misc/cs0501.md

@@ -13,15 +13,42 @@ ms.assetid: 3ff45208-5b9b-42f6-8a12-1eb38a665f33
 'member function' must declare a body because it is not marked abstract, extern, or partial  
 
  Nonabstract methods must have implementations.  
+
+## Explanation
+
+In C#, methods/functions that are a part of a class must have a "body", or implementation. The compiler needs to know what should happen when these methods are called, so that it knows what to execute. A method with no body is not acceptable to the compiler because it wants to avoid confusion about the intent of the code.
+
+There are exceptions to this rule:
+
+* When the method is marked `abstract` as an [Abstract Method](../language-reference/keywords/abstract.md)
+* When the method is marked `extern` as an [External Method](../language-reference/keywords/extern.md)
+* When the method is marked `partial` as a [Partial Method](../language-reference/keywords/partial-method.md)
+
+## Example
+
+The following sample generates CS0501:  
 
- The following sample generates CS0501:  
-  
 ```csharp  
-// CS0501.cs  
-// compile with: /target:library  
-public class clx  
+public class MyClass
+{  
+   public void MethodWithNoBody();   // CS0501 declared but not defined  
+}  
+```
+
+This could be fixed by declaring a body (by adding brackets):
+
+```csharp  
+public class MyClass
+{  
+   public void MethodWithNoBody() { };   // No error; compiler now interprets as an empty method
+}  
+```
+
+Or, using an appropriate keyword, such as defining an `abstract` method:
+
+```csharp
+abstract class MyClass // class is abstract; classes that inherit from it will have to deifne MyAbstractMethod
 {  
-   public void f();   // CS0501 declared but not defined  
-   public void g() {}   // OK  
+   public abstract void MyAbstractMethod();   // Compiler now knows that this method must be deinfed by inheriting classes.
 }  
 ```