Add a note about Higher-Ranked Trait Bounds in docs on Closures.

When using closures that take references with explicit lifetimes sometimes
it's required to use where F: for<..> ... syntax to express the right
lifetimes. This adds a quick note to the docs so other users can discover
it as well.

Author: vvanders

src/doc/book/closures.md

@@ -319,6 +319,53 @@ assert_eq!(3, answer);
 Now we take a trait object, a `&Fn`. And we have to make a reference
 to our closure when we pass it to `call_with_one`, so we use `&||`.
 
+A quick note about closures that use explicit lifetimes. Sometimes you might have a closure
+that takes a reference like so:
+
+```
+fn call_with_ref<F>(some_closure:F) -> i32
+    where F: Fn(&i32) -> i32 {
+
+    let mut value = 0;
+    some_closure(&value)
+}
+```
+
+Normally you can specify the lifetime of the parameter to our closure. We
+could annotate it on the function declaration:
+
+```
+fn call_with_ref<'a, F>(some_closure:F) -> i32 
+    where F: Fn(&'a 32) -> i32 {
+```
+
+However this presents a problem with in our case. When you specify the explict
+lifetime on a function it binds that lifetime to the *entire* scope of the function
+instead of just the invocation scope of our closure. This means that the borrow checker
+will see a mutable reference in the same lifetime as our immutable reference and fail
+to compile.
+
+In order to say that we only need the lifetime to be valid for the invocation scope
+of the closure we can use Higher-Ranked Trait Bounds with the `for<...>` syntax:
+
+```
+fn call_with_ref<F>(some_closure:F) -> i32
+    where F: for<'a> Fn(&'a 32) -> i32 {
+```
+
+This lets the rust compiler find the minimum lifetime to invoke our closure and 
+satisfy the borrow checker's rules. Our function then compiles and excutes as we
+expect.
+
+```
+fn call_with_ref<F>(some_closure:F) -> i32
+    where F: for<'a> Fn(&'a i32) -> i32 {
+
+    let mut value = 0;
+    some_closure(&value)
+}
+```
+
 # Function pointers and closures
 
 A function pointer is kind of like a closure that has no environment. As such,