lib: clarify the doc-comment for struct kr_query::timestamp

(cherry picked from commit 33151c4)

Author: vcunat

lib/rplan.h

@@ -98,7 +98,7 @@ struct kr_query {
 				 * ancestor if it is a subquery. */
 	uint64_t timestamp_mono; /**< Time of query created or time of
 	                           * query to upstream resolver (milliseconds). */
-	struct timeval timestamp; /**< Real time for TTL+DNSSEC checks (.tv_sec only). */
+	struct timeval timestamp; /**< Creation real time.  For TTL+DNSSEC checks we use .tv_sec only. */
 	struct kr_zonecut zone_cut;
 	struct kr_layer_pickle *deferred;