journal: update comments a bit

src/journal/journal-internal.h

@@ -57,9 +57,16 @@ struct Match {
 };
 
 typedef enum LocationType {
+        /* The first and last entries, resp. */
         LOCATION_HEAD,
         LOCATION_TAIL,
+
+        /* We already read the entry we currently point to, and the
+         * next one to read should probably not be this one again. */
         LOCATION_DISCRETE,
+
+        /* We should seek to the precise location specified, and
+         * return it, as we haven't read it yet. */
         LOCATION_SEEK
 } LocationType;