Update System.Random documentation regarding seeds

[mlodato517]

The current documentation for System.Random says:

Initializing two random number generators in a tight loop or in rapid succession creates two random number generators that can produce identical sequences of random numbers.

This is true in .NET Framework but doesn't seem to be true in .NET Core.

Can we update the documentation to highlight this difference? It doesn't matter too much, of course, because the general advice is to not generate lots of System.Randoms, but instead use a single one but 🤷‍♂.

[rpetrusha]

Thanks for pointing this out, @mlodato517. We've opened #3084 to point out that this limitation applies to .NET Framework only.