Add explanation for (copy, clone)_from_slice

It elaborates over why we have to slice the 4-size src to 2-size (same as dst).

Author: anirudhb

src/libcore/slice/mod.rs

@@ -1541,6 +1541,10 @@ impl<T> [T] {
     /// let src = [1, 2, 3, 4];
     /// let mut dst = [0, 0];
     ///
+    /// // Note: the slices must be the same length, so
+    /// // you can slice the source to be the same size.
+    /// // Here we slice the source, four elements, to two, the same size
+    /// // as the destination slice. It *will* panic if we don't do this.
     /// dst.clone_from_slice(&src[2..]);
     ///
     /// assert_eq!(src, [1, 2, 3, 4]);
@@ -1607,6 +1611,10 @@ impl<T> [T] {
     /// let src = [1, 2, 3, 4];
     /// let mut dst = [0, 0];
     ///
+    /// // Note: the slices must be the same length, so
+    /// // you can slice the source to be the same size.
+    /// // Here we slice the source, four elements, to two, the same size
+    /// // as the destination slice. It *will* panic if we don't do this.
     /// dst.copy_from_slice(&src[2..]);
     ///
     /// assert_eq!(src, [1, 2, 3, 4]);