Document unintuitive argument order for Vec::dedup_by relation

When trying to use dedup_by to merge some auxiliary information from
removed elements into kept elements, I was surprised to observe that
vec.dedup_by(same_bucket) calls same_bucket(a, b) where b appears
before a in the vector, and discards a when true is returned.  This
argument order is probably a bug, but since it has already been
stabilized, I guess we should document it as a feature and move on.

(Vec::dedup also uses == with this unexpected argument order, but I
figure that’s not important since == is expected to be symmetric with
no side effects.)

Signed-off-by: Anders Kaseorg <andersk@mit.edu>

src/liballoc/vec.rs

@@ -823,7 +823,8 @@ impl<T> Vec<T> {
         }
     }
 
-    /// Removes consecutive elements in the vector that resolve to the same key.
+    /// Removes all but the first of consecutive elements in the vector that resolve to the same
+    /// key.
     ///
     /// If the vector is sorted, this removes all duplicates.
     ///
@@ -842,11 +843,13 @@ impl<T> Vec<T> {
         self.dedup_by(|a, b| key(a) == key(b))
     }
 
-    /// Removes consecutive elements in the vector according to a predicate.
+    /// Removes all but the first of consecutive elements in the vector satisfying a given equality
+    /// relation.
     ///
     /// The `same_bucket` function is passed references to two elements from the vector, and
-    /// returns `true` if the elements compare equal, or `false` if they do not. Only the first
-    /// of adjacent equal items is kept.
+    /// returns `true` if the elements compare equal, or `false` if they do not. The elements are
+    /// passed in opposite order from their order in the vector, so if `same_bucket(a, b)` returns
+    /// `true`, `a` is removed.
     ///
     /// If the vector is sorted, this removes all duplicates.
     ///