Missing doc on whether compactRange() is synchronous or asynchronous

[bowenli86]

Missing documentation on explain whether compactRange() is synchronous or asynchronous. It's hard for users to figure this out themselves. I spend a couple hours on it, and still not sure about the answer.

[sagar0]

Thanks for posting your question in the RocksDB dev facebook group as well. Feel free to update the documentation once you figure out the correct behavior.

[ajkr]

it's synchronous, for the record