Documentation for Async.CancellationToken and Async.DefaultCancellationToken shows .NET supported version information #250

The current documentation for Async.CancellationToken and Async.DefaultCancellationToken shows that it is available in F# 2.0, not just F# 4.0 and portable (this information is under the section of "Version Information").

For example: https://msdn.microsoft.com/en-us/visualfsharpdocs/conceptual/async.defaultcancellationtoken-property-%5bfsharp%5d

CancellationToken is not supported in F# 2.0.

The long explanation I think is this: it is actually only supported in F# 3.0, F# 4.0 and PCL targeted for .NET 4 and above, because CancellationToken is only available in .NET 4 and AFAIK .NET 4 is supported by F# since F# 3.0, not since F# 2.0.

Please correct me if I'm wrong. If this issue is correct in a sense that we should clearly change it, If this issue is confirmed to be correct that the version information is still wrong, I will submit a PR for this.

cc Den (@dend) , Phillip Carter (@cartermp)