Missing remark about only works with public classes

ComVisible is ignored if the class it's on is not public which seems like something the docs should call out.

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* ID: cb7b7d06-51ce-e55a-7ec9-bc146f746793
* Version Independent ID: fa60b098-75bc-f5d1-fe82-3565497d249c
* Content: [ComVisibleAttribute(Boolean) Constructor (System.Runtime.InteropServices)](https://docs.microsoft.com/en-us/dotnet/api/system.runtime.interopservices.comvisibleattribute.-ctor?redirectedfrom=MSDN&view=netframework-4.7.2#System_Runtime_InteropServices_ComVisibleAttribute__ctor_System_Boolean_)
* Content Source: [xml/System.Runtime.InteropServices/ComVisibleAttribute.xml](https://github.com/dotnet/dotnet-api-docs/blob/master/xml/System.Runtime.InteropServices/ComVisibleAttribute.xml)
* Product: **dotnet-api**
* GitHub Login: @dotnet-bot
* Microsoft Alias: **dotnetcontent**