Add AddWebAllowedObject to unsupported API list

[rjmurillo]

Issue: #

MicrosoftDocs/WindowsCommunityToolkitDocs#72

PR Type

What kind of change does this PR introduce?

• Documentation content changes

What is the current behavior?

The known issues does not include information about a missing WinRT API AddWebAllowedObjects(String, Object) which is not supported.

What is the new behavior?

Documentation includes the unsupported API next to information of the related ObjectForScripting

PR Checklist

Please check if your PR fulfills the following requirements:

• Tested code with current supported SDKs
• Docs have been added/updated which fit documentation template. (for bug fixes / features)
• Sample in sample app has been added / updated (for bug fixes / features)
  • Icon has been created (if new sample) following the Thumbnail Style Guide and templates
• Tests for the changes have been added (for bug fixes / features) (if applicable)
• Header has been added to all new source files (run build/UpdateHeaders.bat)
• Contains NO breaking changes

Other information

CC: @normesta @ericlaw1979

[ericlaw1979]

LGTM, thanks for the fix!