Rollup merge of rust-lang#51701 - anirudhb:master, r=frewsxcv

Better docs for copy_from_slice & clone_from_slice

I copy-pasted the text from clone_from_slice to copy_from_slice 😄

@steveklabnik feel free to suggest changes.

edit: closes rust-lang#49769

Author: GuillaumeGomez

src/libcore/slice/mod.rs

@@ -1541,6 +1541,9 @@ impl<T> [T] {
     /// let src = [1, 2, 3, 4];
     /// let mut dst = [0, 0];
     ///
+    /// // Because the slices have to be the same length,
+    /// // we slice the source slice from four elements
+    /// // to two. It will panic if we don't do this.
     /// dst.clone_from_slice(&src[2..]);
     ///
     /// assert_eq!(src, [1, 2, 3, 4]);
@@ -1607,6 +1610,9 @@ impl<T> [T] {
     /// let src = [1, 2, 3, 4];
     /// let mut dst = [0, 0];
     ///
+    /// // Because the slices have to be the same length,
+    /// // we slice the source slice from four elements
+    /// // to two. It will panic if we don't do this.
     /// dst.copy_from_slice(&src[2..]);
     ///
     /// assert_eq!(src, [1, 2, 3, 4]);