mmaprototype: explain why localities don't care about the keys

It's because operators must use matching keys across the cluster,
so only the values can be different.

For example, it's not allowed to have one node with `region=foo` and another
with `fruit=banana`. It must be `region=foo`,`region=banana` and that point the
key no longer plays a role since it always matches.

Epic: CRDB-55052

Author: tbg

pkg/kv/kvserver/allocator/mmaprototype/constraint.go

@@ -1922,6 +1922,14 @@ func (lti *localityTierInterner) unintern(lt localityTiers) roachpb.Locality {
 	return locality
 }
 
+// localityTiers encodes a locality value hierarchy, represented by codes
+// from an associated stringInterner.
+//
+// Note that CockroachDB operators must use matching *keys*[1] across all nodes
+// in each deployment, so we only need to deal with a slice of locality
+// *values*.
+//
+// [1]: https://www.cockroachlabs.com/docs/stable/cockroach-start#locality
 type localityTiers struct {
 	tiers []stringCode
 	// str is useful as a map key for caching computations.