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.

README.md

@@ -544,10 +544,10 @@ bash scripts/selftest.sh --fast   # fixture-only suites, no compiler
 
 Both run in CI — `.gitea/workflows/ci.yaml`, one job, on pushes to `main` and
 on PRs that were non-draft when the run was created. It installs shellcheck
-and both a stable and a nightly Rust toolchain (nightly for
-`-Z checksum-freshness`, without which `hardlink-clone-selftest.sh` skips the
-scenario that covers the silent-stale-reuse hazard) and references no
-credentials; the scratch
+and both a stable and a nightly Rust toolchain (nightly so
+`hardlink-clone-selftest.sh` can run its content-freshness scenario — which no
+nightly currently enables, so it is skipped and the step is kept only against
+the day upstream restores it) and references no credentials; the scratch
 workspaces the compiler-backed suites build use path dependencies only, so
 nothing reaches crates.io. It runs the full suite rather than `--fast`,
 because the two compiler-backed suites are the ones that check this scheme