Update diagnostic ID CP0012 description (#44889)

* Update diagnostic ID CP0012 description

* Update docs/fundamentals/apicompat/diagnostic-ids.md

Co-authored-by: Genevieve Warren <[email protected]>

* Update docs/fundamentals/apicompat/diagnostic-ids.md

Co-authored-by: Genevieve Warren <[email protected]>

---------

Co-authored-by: Genevieve Warren <[email protected]>

Author: carlossanlop

docs/fundamentals/apicompat/diagnostic-ids.md

@@ -61,7 +61,7 @@ This reference article lists all the error codes generated by package validation
 | CP0009 | A type that was unsealed on one side was annotated as `sealed` on the other compared side. | Remove the `sealed` annotation from the type. |
 | CP0010 | The underlying type of an enum changed from one side to the other. | Change the underlying type back to what it was previously. |
 | CP0011 | The value of a member in an enum changed from one side to the other. | Change the value of the member back to what it was previously. |
-| CP0012 | The `virtual` keyword was removed from a member that was previously virtual. | Add the `virtual` keyword back to the member. |
+| CP0012 | Either the `virtual` or `abstract` keyword was removed from a member that was previously virtual or abstract. | If the member was previously virtual, add the `virtual` keyword back. If the member was previously abstract, add either the `virtual` or `abstract` keyword to the member. |
 | CP0013 | The `virtual` keyword was added to a member that was previously not virtual. | Remove the `virtual` keyword from the member. |
 | CP0014 | An attribute was removed from a member that previously had it. | Add the attribute back to the member. |
 | CP0015 | The arguments passed to an attribute changed from one side to the other. | Change the arguments to the attribute back to what they were previously. |