Add link to collections docs to extend trait

library/std/src/collections/mod.rs

@@ -172,7 +172,8 @@
 //!
 //! ## Iterators
 //!
-//! Iterators are a powerful and robust mechanism used throughout Rust's
+//! [Iterators][crate::iter]
+//! are a powerful and robust mechanism used throughout Rust's
 //! standard libraries. Iterators provide a sequence of values in a generic,
 //! safe, efficient and convenient way. The contents of an iterator are usually
 //! *lazily* evaluated, so that only the values that are actually needed are
@@ -252,7 +253,9 @@
 //!
 //! Several other collection methods also return iterators to yield a sequence
 //! of results but avoid allocating an entire collection to store the result in.
-//! This provides maximum flexibility as `collect` or `extend` can be called to
+//! This provides maximum flexibility as
+//! [`collect`][crate::iter::Iterator::collect] or
+//! [`extend`][crate::iter::Extend::extend] can be called to
 //! "pipe" the sequence into any collection if desired. Otherwise, the sequence
 //! can be looped over with a `for` loop. The iterator can also be discarded
 //! after partial use, preventing the computation of the unused items.