Merge pull request #29997 from smasher164/virtual-diagnostics

add docs for diagnostics CP0012 and CP0013

Author: smasher164

docs/fundamentals/package-validation/diagnostic-ids.md

@@ -30,6 +30,8 @@ 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. |
+| CP0013 | The virtual keyword was added to a member that was previously not virtual. | Remove the virtual keyword from the member. |
 | CP1001 | A matching assembly could not be found in the search directories. (Not applicable for Package Validation, only when using API Compat directly.) | Provide the search directory when loading matching assemblies using the `AssemblySymbolLoader`. |
 | CP1002 | A reference assembly was not found when loading the assemblies to compare in the resolved directories for the current target framework. | Include the directory path where that assembly can be found using the following MSBuild item: `<PackageValidationReferencePath Include="<path>" TargetFramework="<tfm>" />`. |
 | CP1003 | There was no search directory provided for the target framework moniker that the package validation is running API Compat for. | Provide the search directory to find references for that target framework using the following MSBuild item: `<PackageValidationReferencePath Include="<path>" TargetFramework="<tfm>" />`.