Extend the description of the DataGridCellInfo.Equality to indicate that the result is containers virtualization depended.

Documentation for DataGridCellInfo.[Equality](https://docs.microsoft.com/en-us/dotnet/api/system.windows.controls.datagridcellinfo.op_equality?view=netcore-3.1) not point that fact that equality test can return false when the items are the same but the containers are different due virtualization.

The proposal is to include this information in description of DataGridCellInfo.Equality operator.

Full discussion is [here](https://github.com/dotnet/wpf/issues/3297#issue-670253657).