Add documentation about for used as higher ranked trait bounds

src/libstd/keyword_docs.rs

@@ -286,11 +286,13 @@ mod fn_keyword { }
 //
 /// The `for` keyword.
 ///
-/// `for` is primarily used in for-in-loops, but it has a few other pieces of syntactic uses such as
-/// `impl Trait for Type` (see [`impl`] for more info on that). for-in-loops, or to be more
-/// precise, iterator loops, are a simple syntactic sugar over an exceedingly common practice
-/// within Rust, which is to loop over an iterator until that iterator returns `None` (or `break`
-/// is called).
+/// `for` is primarily used in for-in-loops, but it has a few other pieces of syntactic
+/// uses.  `for` is used when implementing traits as in `impl Trait for Type` (see
+/// [`impl`] for more info on that).  `for` is also used for [higher-ranked trait bounds]
+/// as in `for<'a> &'a T: PartialEq<i32>`.  for-in-loops, or to be more precise, iterator
+/// loops, are a simple syntactic sugar over an exceedingly common practice within Rust,
+/// which is to loop over an iterator until that iterator returns `None` (or `break` is
+/// called).
 ///
 /// ```rust
 /// for i in 0..5 {