docs: clarify revisionHistoryLimit applies to Knative mode

[nelonmelons]

What

Corrects the spec.revisionHistoryLimit documentation, which still says "Only used in ReplicaSet mode" even though the Knative reconciler now honours it.

Why

While chasing idle pods from accumulated Knative revisions I filed #182, believing there was no operator-side retention for Knative Configurations. Reading main more carefully, that turned out to be wrong: cleanup_old_configurations already bounds retained Configurations with spec.revision_history_limit (added recently in 0459f15, "Cleanup: fix deletion hang"):

// src/controllers/restatedeployment/reconcilers/knative.rs
if retain_for_rollback(mode, historic_count, rsd.spec.revision_history_limit) {
    historic_count += 1;
    // ... keep within limit
}
// else: deregister from Restate + delete the Configuration

The field doc, however, still reads "The number of old ReplicaSets to retain to allow rollback. Only used in ReplicaSet mode." That description is what led me (and I suspect others) to believe the limit does nothing in Knative mode, so old drained Configurations look unbounded.

Change

Documentation only. Rewords the field on the struct and mirrors it into the generated crd/restatedeployments.yaml and crd/RestateDeployment.pkl to say it applies to both modes, and adds the Knative-specific caveat: a retained Configuration keeps running its minScale pods (Knative cannot rescale an existing Revision), so services with a high minScale should lower this to avoid idle pods held only for rollback.

Note on generated files

I don't have cargo / pkl available in my environment, so crd/restatedeployments.yaml and crd/RestateDeployment.pkl were updated by hand to match the struct doc comment. Please run just generate / just generate-pkl to confirm the exact formatting matches. No Rust logic changed, so cargo clippy / cargo test are unaffected.

Possible follow-up (not in this PR)

The default of 10 is cheap for ReplicaSets (scaled to zero) but expensive in Knative, where each retained Configuration holds minScale pods. A lower Knative default, or scaling non-latest retained Configurations down, might be worth considering. Happy to follow up if that's of interest.

Refs #182.

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