Update BTreeMap recommendation

Focus on the ordering/range benefit.

Author: arthurprs

src/libstd/collections/mod.rs

@@ -64,11 +64,11 @@
 //! * You want a map, with no extra functionality.
 //!
 //! ### Use a `BTreeMap` when:
+//! * You want a map sorted by its keys.
+//! * You want to be able to get a range of entries on-demand.
 //! * You're interested in what the smallest or largest key-value pair is.
 //! * You want to find the largest or smallest key that is smaller or larger
 //!   than something.
-//! * You want to be able to get all of the entries in order on-demand.
-//! * You want a map sorted by its keys.
 //!
 //! ### Use the `Set` variant of any of these `Map`s when:
 //! * You just want to remember which keys you've seen.