Update BTreeSet::drain_filter documentation

This commit makes the documentation of `BTreeSet::drain_filter` more
consistent with that of `BTreeMap::drain_filter` after the changes in
f0b8166.

In particular, this explicitly documents the iteration order.

library/alloc/src/collections/btree/set.rs

@@ -941,18 +941,20 @@ impl<T> BTreeSet<T> {
         BTreeSet { map: self.map.split_off(key) }
     }
 
-    /// Creates an iterator which uses a closure to determine if a value should be removed.
+    /// Creates an iterator that visits all values in ascending order and uses a closure
+    /// to determine if a value should be removed.
     ///
-    /// If the closure returns true, then the value is removed and yielded.
-    /// If the closure returns false, the value will remain in the list and will not be yielded
-    /// by the iterator.
+    /// If the closure returns `true`, the value is removed from the set and yielded. If
+    /// the closure returns `false`, or panics, the value remains in the set and will
+    /// not be yielded.
     ///
-    /// If the iterator is only partially consumed or not consumed at all, each of the remaining
-    /// values will still be subjected to the closure and removed and dropped if it returns true.
+    /// If the iterator is only partially consumed or not consumed at all, each of the
+    /// remaining values is still subjected to the closure and removed and dropped if it
+    /// returns `true`.
     ///
-    /// It is unspecified how many more values will be subjected to the closure
-    /// if a panic occurs in the closure, or if a panic occurs while dropping a value, or if the
-    /// `DrainFilter` itself is leaked.
+    /// It is unspecified how many more values will be subjected to the closure if a
+    /// panic occurs in the closure, or if a panic occurs while dropping a value, or if
+    /// the `DrainFilter` itself is leaked.
     ///
     /// # Examples
     ///