Merge pull request dotnet#31738 from smasher164/visibility-diagnostic-id

add docs for diagnostics CP0019 and CP0020

Author: smasher164

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

@@ -37,6 +37,8 @@ This reference article lists all the error codes generated by package validation
 | CP0016 | An attribute was added to a member that previously did not have it. | Remove the attribute from the member. |
 | CP0017 | The name of a method's parameter changed from one side to the other. | Change the parameter's name back to what it was previously. |
 | CP0018 | The `sealed` keyword was added to an interface member that was previously not sealed. | Remove the `sealed` keyword from the interface member. |
+| CP0019 | The visibility of a member was reduced from one side to the other. | Change the member's visibility back to what it was previously. |
+| CP0020 | The visibility of a member was expanded from one side to the other. | Change the member's visibility back to what it was previously. |
 | 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 `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>" />`.