docs(ci): say what the nightly step actually buys today

The step's comment claimed nightly was not optional. CI's own first green run
falsified that: 1.100.0-nightly accepts -Z checksum-freshness and resolves
freshness by mtime regardless, so the suite's probe reports it and skips the
scenario either way.

The step stays — twenty seconds, and the coverage returns by itself the day
upstream restores the behaviour — but the comment now says that rather than
the opposite. The open question about what upstream actually did is
daniel/gitdan#62.

.gitea/workflows/ci.yaml

@@ -69,13 +69,16 @@ jobs:
       # Nightly first, stable second, so stable ends up the default and
       # nightly is only reachable through an explicit `+nightly`.
       #
-      # Nightly is not optional here. `hardlink-clone-selftest.sh` gates its
-      # two strongest assertions on `-Z checksum-freshness` — the mode where
-      # Cargo's dep-info file carries per-source checksums and is rewritten in
-      # place, which is the mutation that turns a hardlink clone into SILENT
-      # stale-artifact reuse rather than a slow build. Without a nightly the
-      # suite still runs, and skips exactly the hazard this whole scheme exists
-      # to close.
+      # `hardlink-clone-selftest.sh`'s last scenario needs a Cargo that
+      # resolves freshness by CONTENT — the mode where the dep-info file
+      # carries per-source checksums, which is the mutation that turns a
+      # hardlink clone into silent stale-artifact reuse rather than a slow
+      # build. As of 1.100.0-nightly (2026-08-25) no nightly provides it:
+      # `-Z checksum-freshness` is still accepted and freshness is still
+      # resolved by mtime, so the suite's probe reports that by name and skips
+      # the scenario. This step therefore buys nothing today and is kept
+      # anyway — it costs about twenty seconds, and the day upstream restores
+      # the behaviour the coverage comes back with no edit here. See daniel/gitdan#62.
       - name: Install Rust nightly
         uses: dtolnay/rust-toolchain@nightly
       - name: Install Rust toolchain