Clarify how normal modules are located inside a `#[path]` module.

[ehuss]

The path attribute documentation explains how #[path] annotated modules are found. It does not explain how non-attributed modules are located once inside a #[path] module. For example:

// lib.rs
#[path = "foo.rs"]
mod x;

// foo.rs
mod y;   // y must be y.rs or y/mod.rs, relative to foo.rs.

In this case, once inside foo.rs, module locations will be relative to foo's location.