fix: make the rust-toolchain.toml comment match what it actually does

[MaximeGaudin]

Summary

• The file's comment claimed "Pin the Rust toolchain so every developer and CI use the same rustc/clippy", but channel = "stable" is a floating channel, not a pin.
• Rewrote the comment to state what the file actually does: it selects the current stable channel (unpinned, resolved per machine) with rustfmt and clippy.

The issue offered "pin to a real version or correct the comment". Pinning to a specific version is a toolchain-policy decision (which version, and how it interacts with CI — see below) that I left to the maintainer; this PR does the unambiguous half.

Finding worth a follow-up: the file currently overrides CI's toolchain selection

Verified locally with rustup: the toolchain file takes precedence over rustup default (rustup precedence: RUSTUP_TOOLCHAIN > directory override > toolchain file > default). The dtolnay/rust-toolchain action installs its @rev toolchain and sets it with rustup default — which this file then overrides, because channel = "stable" resolves to the current stable. Consequence: in CI, every job — including the msrv job that intends 1.95.0 — actually runs on whatever stable is current at run time, so the MSRV floor is not being tested. (Repro: in a dir with this file present, rustup default 1.95.0 + rustc --version reports "overridden by rust-toolchain.toml" and uses stable.)

If the maintainer wants, the clean follow-up is to drop the channel line (leaving components/profile), which lets each CI job's @rev take effect as intended — at the cost of local developers using their own default toolchain instead of a forced stable.

Fixes #223

Test plan

• Comment-only change; no code affected, nothing to run (verified rustup precedence empirically before writing the note).
• CI green on the PR