Add section: What can I do to a Workflow?

[lorensr]

I think it would be helpful to see a list of things I can do to a Workflow Execution, what they do, and why I might want to do them.

• start
• get result
• describe
• query
• signal
• cancel
• terminate

Some info to include:

• What Cancelling does (& note that you can't request cancellation multiple times)
• When to Cancel vs Terminate