kvpb: update comment on ProbeRequest

pkg/kv/kvpb/api.proto

@@ -249,14 +249,15 @@ message GetResponse {
   Value intent_value = 3;
 }
 
-// A ProbeRequest is an internal request type used to send a replicated
-// no-op through a Range as a means of probing write availability. The
-// request will be serialized like a regular write, i.e. will acquire
-// latches, and declare key access, but it will not check locks (i.e.
-// if an intent exists on the key that is being probed, the probe will
-// not observe it). ProbeRequest can be served by any Replica including
-// followers, i.e. it can be used to verify that a given Replica is able
-// to access the replication layer.
+// A ProbeRequest is an internal request type used to send a replicated no-op
+// through a Range as a means of probing write availability. The request does
+// not take any latches or locks and thus is not serialized like a regular
+// write. This is because we want the probe to bypass as much of the above-raft
+// machinery as possible so that it gives us a signal on the replication layer
+// alone.
+//
+// ProbeRequest can be served by any Replica including followers, i.e. it can be
+// used to verify that a given Replica is able to access the replication layer.
 message ProbeRequest {
   RequestHeader header = 1 [(gogoproto.nullable) = false, (gogoproto.embed) = true];
 }