Wm notes 1.15.0

Pinging @mglukhovsky (and @larkost if he wants to check my checking on pull requests and issues).
This paragraph could be clarified. I suggest:

> The performance improvements will only be in effect when reading from documents that have been modified since upgrading to 1.15. You can force RethinkDB to "upgrade" your older documents by performing any write on them (except a no-op). For example, you could add a dummy field and then remove it: