Document that summary's order should be compatible

At time of writing, I'm not sure about the exact relationship here, but
there definitely ought to be one!

timely/src/progress/reachability.rs

@@ -205,7 +205,7 @@ impl<T: Timestamp> Builder<T> {
         Tracker::allocate_from(self, logger)
     }
 
-    /// Tests whether the graph a cycle of default path summaries.
+    /// Tests whether the graph includes a cycle of default path summaries.
     ///
     /// Graphs containing cycles of default path summaries will most likely
     /// not work well with progress tracking, as a timestamp can result in

timely/src/progress/timestamp.rs

@@ -31,6 +31,10 @@ pub trait PathSummary<T> : Clone+'static+Eq+PartialOrder+Debug+Default {
     /// in computation, uses this method and will drop messages with timestamps that when advanced
     /// result in `None`. Ideally, all other timestamp manipulation should behave similarly.
     ///
+    /// This summary's partial order is expected to be compatible with the partial order of [T],
+    /// in the sense that if `s1.less_equal(s2)`, then `s1.results_in(&t)` is less than or equal to
+    /// `s2.results_in(&t)`.
+    ///
     /// Note that `Self::default()` is expected to behave as an "empty" or "noop" summary, such that
     /// `Self::default().results_in(&t) == Some(t)`.
     ///