Document content hash .watchmanconfig options

We recently bumped into the issue described in https://github.com/facebook/watchman/issues/737 at Stripe, and I found @wez's responses describing `content_hash_max_items` and friends to be helpful. These tuning options do not appear to be documented, however:

https://facebook.github.io/watchman/docs/config.html

It would be wonderful if you could refresh this page to document these options (and any other useful config options)! :)