Remove outdated note in the associations module doc
Closes
//! let data: Vec<(User, Post)> = users::table.inner_join(posts::table).load(&connection);
//! ```
//!
-//! Note: Due to language limitations, only two tables can be joined per query. This will change in
-//! the future.
-//!
//! Typically however, queries are loaded in multiple queries. For most datasets, the reduced
//! amount of duplicated information sent over the wire saves more time than the extra round trip
//! costs us. You can load the children for a single parent using the
