Update AlphaOption.cs xml docs

[kmgallahan]

This copies over the descriptions from the official docs.

Description

I think adding the full descriptions here would be useful. Although it does present a potential source of maintenance if things change.

Good or bad to update the xml comments like this in some places?

• Discrete has no documentation
• On/Off don't either, although they are somewhat self-explanatory
• Undefined has... undefined behavior
• Associate/Disassociate docs are somewhat ambiguous compared to the others

[dlemstra]

Thanks for improving the documentation and feel free to add more of these kind of changes.