Mention what iterator terminators do with an empty iterator

src/libcore/iter.rs

@@ -1565,6 +1565,8 @@ fn fold<B, F>(self, init: B, mut f: F) -> B where
     /// as soon as it finds a `false`, given that no matter what else happens,
     /// the result will also be `false`.
     ///
+    /// An empty iterator returns `true`.
+    ///
     /// # Examples
     ///
     /// Basic usage:
@@ -1613,6 +1615,8 @@ fn all<F>(&mut self, mut f: F) -> bool where
     /// as soon as it finds a `true`, given that no matter what else happens,
     /// the result will also be `true`.
     ///
+    /// An empty iterator returns `false`.
+    ///
     /// # Examples
     ///
     /// Basic usage:
@@ -2071,6 +2075,8 @@ fn cycle(self) -> Cycle<Self> where Self: Sized + Clone {
     ///
     /// Takes each element, adds them together, and returns the result.
     ///
+    /// An empty iterator returns the zero value of the type.
+    ///
     /// # Examples
     ///
     /// Basic usage:
@@ -2094,6 +2100,8 @@ fn sum<S=<Self as Iterator>::Item>(self) -> S where
 
     /// Iterates over the entire iterator, multiplying all the elements
     ///
+    /// An empty iterator returns the one value of the type.
+    ///
     /// # Examples
     ///
     /// ```