Rollup merge of rust-lang#29539 - shepmaster:empty-iterator-docs, r=Gankro

Author: steveklabnik

src/libcore/iter.rs

@@ -1565,6 +1565,8 @@ pub trait Iterator {
     /// 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 @@ pub trait Iterator {
     /// 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 @@ pub trait Iterator {
     ///
     /// 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 @@ pub trait Iterator {
 
     /// Iterates over the entire iterator, multiplying all the elements
     ///
+    /// An empty iterator returns the one value of the type.
+    ///
     /// # Examples
     ///
     /// ```