add to wasm-tools validate docs

[michelleN]

• Adds where to find available wasm feature options
• Adds how to see more verbose output in case folks want to see more details of what is being validated

[alexcrichton]

Thanks!

If you're feeling particularly intrepid the error message here could theoretically be improved with something like "didn't recognize foo but valid features are bar, baz, ...". Or something like --wasm-features help would print them all out or something like that. In any case no need to happen here-and-now, just if you're interested!