Explicitely spell out behaviour on overflow for usize-returning iterators

Mention that panics are guaranteed if debug assertions are active, otherwise a
wrong result might be returned.

Author: tbu-

src/libcore/iter.rs

@@ -106,10 +106,12 @@ pub trait Iterator {
 
     /// Counts the number of elements in this iterator.
     ///
-    /// # Undefined overflow
+    /// # Overflow Behavior
     ///
     /// The method does no guarding against overflows, so counting elements of
-    /// an iterator with more than `usize::MAX` elements is undefined.
+    /// an iterator with more than `usize::MAX` elements either produces the
+    /// wrong result or panics. If debug assertions are enabled, a panic is
+    /// guaranteed.
     ///
     /// # Panics
     ///
@@ -291,10 +293,11 @@ pub trait Iterator {
     /// different sized integer, the `zip` function provides similar
     /// functionality.
     ///
-    /// # Undefined overflow
+    /// # Overflow Behavior
     ///
     /// The method does no guarding against overflows, so enumerating more than
-    /// `usize::MAX` elements is undefined.
+    /// `usize::MAX` elements either produces the wrong result or panics. If
+    /// debug assertions are enabled, a panic is guaranteed.
     ///
     /// # Panics
     ///
@@ -695,11 +698,12 @@ pub trait Iterator {
     ///
     /// Does not consume the iterator past the first found element.
     ///
-    /// # Undefined overflow
+    /// # Overflow Behavior
     ///
-    /// The method does no guarding against overflows, so when there are more
-    /// than `usize::MAX` non-matching elements, the overflow behaviour is
-    /// undefined.
+    /// The method does no guarding against overflows, so if there are more
+    /// than `usize::MAX` non-matching elements, it either produces the wrong
+    /// result or panics. If debug assertions are enabled, a panic is
+    /// guaranteed.
     ///
     /// # Panics
     ///
@@ -1801,10 +1805,11 @@ pub struct Enumerate<I> {
 impl<I> Iterator for Enumerate<I> where I: Iterator {
     type Item = (usize, <I as Iterator>::Item);
 
-    /// # Undefined overflow
+    /// # Overflow Behavior
     ///
     /// The method does no guarding against overflows, so enumerating more than
-    /// `usize::MAX` elements is undefined.
+    /// `usize::MAX` elements either produces the wrong result or panics. If
+    /// debug assertions are enabled, a panic is guaranteed.
     ///
     /// # Panics
     ///