docs(core): tighten Rust group rationale to reflect measured shape

The initial comment implied `tests.rs` was the load-bearing pattern.
Measurement across 10 major public Rust repos (rust-lang/rust, tokio,
cargo, ripgrep, alacritty, helix, deno, solana, foundry, polkadot-sdk)
showed the actual distribution is bimodal:

  - Library-scale crates keep tests inline via `#[cfg(test)] mod`,
    which no file-pattern rule can touch → group is a near-noop.
  - Workspace monorepos colocate `foo_test.rs` beside `foo.rs` at
    scale → `*_test.rs` alone accounts for the vast majority of hits
    (232 in polkadot-sdk, 253 in rust-lang/rust even with truncated
    tree).

Rewrite the block comment so a future reader lands on the right mental
model — the extracted `tests.rs` form is legitimate but rare; the win
comes from the colocated shape in workspace-heavy repos.