Document implementing try_fold for iterators for internal iteration

src/libcore/iter/mod.rs

@@ -141,9 +141,10 @@
 //! next.
 //!
 //! Also note that `Iterator` provides a default implementation of methods such as `nth` and `fold`
-//! which call `next` internally. However, it is also possible to write a custom implementation of
-//! methods like `nth` and `fold` if an iterator can compute them more efficiently without calling
-//! `next`.
+//! which call `next` internally.  Each of these methods delegates to the `try_fold` implementation.
+//! So if you can produce a more efficient custom implementation of iterating over the entire
+//! structure, then you can implement `try_fold` and get these other methods to use your iteration
+//! strategy.
 //!
 //! # for Loops and IntoIterator
 //!