Wm notes 1.15.0

Pinging @mglukhovsky (and @larkost if he wants to check my checking on pull requests and issues).
We should note that it's not actually necessary for users to do anything if they don't need the performance improvements.

Ideally we can update each document in the table with a no-op. It would look something like `r.table(...).update(r.row)`. (And that's really nice because users can still run queries normally during the operation.) But that doesn't actually work because RethinkDB will recognize that each row has not actually been changed and so it won't rewrite the row to disk. We could add a magic command that does the right thing, and tell people to run it on their tables before the next upgrade, just like secondary index migration. (This would also allow us to remove support for the old datum serialization format in 1.16.) But that change would take some work.