Flesh out database examples in the API documentation:

* Demonstrate multiple databases in ROCKET_DATABASES environment variable.
  * Add a second example of connection guard type creation.

contrib/lib/src/databases.rs

@@ -177,7 +177,14 @@
 //! guide](https://rocket.rs/v0.4/guide/configuration/#environment-variables):
 //!
 //! ```bash
-//! ROCKET_DATABASES={my_db={url="db.sqlite"}}
+//! ROCKET_DATABASES='{my_db={url="db.sqlite"}}'
+//! ```
+//!
+//! Multiple databases can be specified in the `ROCKET_DATABASES` environment variable
+//! as well by comma separating them:
+//!
+//! ```bash
+//! ROCKET_DATABASES='{my_db={url="db.sqlite"},my_pg_db={url="postgres://root:root@localhost/my_pg_db"}}'
 //! ```
 //!
 //! ## Guard Types
@@ -203,6 +210,21 @@
 //! # }
 //! ```
 //!
+//! Other databases can be used by specifying their respective [`Poolable`]
+//! type:
+//!
+//! ```rust
+//! # extern crate rocket;
+//! # #[macro_use] extern crate rocket_contrib;
+//! # #[cfg(feature = "postgres_pool")]
+//! # mod test {
+//! use rocket_contrib::databases::postgres;
+//!
+//! #[database("my_pg_db")]
+//! struct MyPgDatabase(postgres::Connection);
+//! # }
+//! ```
+//!
 //! The macro generates a [`FromRequest`] implementation for the decorated type,
 //! allowing the type to be used as a request guard. This implementation
 //! retrieves a connection from the database pool or fails with a