Michel 228 apisection

@atnnn -- can you take a quick look?

For `filter`, we used to have the description of the `Joins` api section. This commit fixes that.
I also tried the output with 1.12 in the data explorer, and everything seems to work.

Once you merged it, you can regenerate the `reql_docs.json` file, and push it to the rethinkdb repo.
And that should close https://github.com/rethinkdb/rethinkdb/issues/1982