Expand fixture documentation a little
//! A fixture without metadata is parsed into a single source file.
//! Use this to test functionality local to one file.
//!
-//! Example:
+//! Simple Example:
//! ```
//! r#"
//! fn main() {
//! The basic form is specifying filenames,
//! which is also how to define multiple files in a single test fixture
//!
-//! Example:
+//! Example using two files in the same crate:
//! ```
//! "
//! //- /main.rs
//! "
//! ```
//!
+//! Example using two crates with one file each, with one crate depending on the other:
+//! ```
+//! r#"
+//! //- /main.rs crate:a deps:b
+//! fn main() {
+//! b::foo();
+//! }
+//! //- /lib.rs crate:b
+//! pub fn b() {
+//! println!("Hello World")
+//! }
+//! "#
+//! ```
+//!
//! Metadata allows specifying all settings and variables
//! that are available in a real rust project:
//! - crate names via `crate:cratename`
//! - configuration settings via `cfg:dbg=false,opt_level=2`
//! - environment variables via `env:PATH=/bin,RUST_LOG=debug`
//!
-//! Example:
+//! Example using all available metadata:
//! ```
//! "
//! //- /lib.rs crate:foo deps:bar,baz cfg:foo=a,bar=b env:OUTDIR=path/to,OTHER=foo
