Rollup merge of rust-lang#25144 - killercup:docs/iter-fold-reduce, r=steveklabnik

Enhance Google-ability of `.fold()` by mentioning 'reduce' and 'inject' in the docs.

Motivation: [This thread on users.rust-lang.org](https://users.rust-lang.org/t/find-the-shortest-string-in-a-vector/1247)

Author: steveklabnik

src/libcore/iter.rs

@@ -602,6 +602,8 @@ pub trait Iterator {
     /// Performs a fold operation over the entire iterator, returning the
     /// eventual state at the end of the iteration.
     ///
+    /// This operation is sometimes called 'reduce' or 'inject'.
+    ///
     /// # Examples
     ///
     /// ```