Rollup merge of rust-lang#133790 - HypheX:improve-vec-docs, r=harudagondi,workingjubilee

Improve documentation for Vec::extend_from_within

This closes rust-lang#104762.

It rephrases some of the explanations, and greatly improves the clarity of the example.
Based on this PR and its discussions:
https://github.com/rust-lang/rust/pull/105030/files#r1059808792

Author: matthiaskrgr

alloc/src/vec/mod.rs

@@ -3025,26 +3025,29 @@ impl<T: Clone, A: Allocator> Vec<T, A> {
         self.spec_extend(other.iter())
     }
 
-    /// Copies elements from `src` range to the end of the vector.
+    /// Given a range `src`, clones a slice of elements in that range and appends it to the end.
+    ///
+    /// `src` must be a range that can form a valid subslice of the `Vec`.
     ///
     /// # Panics
     ///
-    /// Panics if the starting point is greater than the end point or if
-    /// the end point is greater than the length of the vector.
+    /// Panics if starting index is greater than the end index
+    /// or if the index is greater than the length of the vector.
     ///
     /// # Examples
     ///
     /// ```
-    /// let mut vec = vec![0, 1, 2, 3, 4];
-    ///
-    /// vec.extend_from_within(2..);
-    /// assert_eq!(vec, [0, 1, 2, 3, 4, 2, 3, 4]);
+    /// let mut characters = vec!['a', 'b', 'c', 'd', 'e'];
+    /// characters.extend_from_within(2..);
+    /// assert_eq!(characters, ['a', 'b', 'c', 'd', 'e', 'c', 'd', 'e']);
     ///
-    /// vec.extend_from_within(..2);
-    /// assert_eq!(vec, [0, 1, 2, 3, 4, 2, 3, 4, 0, 1]);
+    /// let mut numbers = vec![0, 1, 2, 3, 4];
+    /// numbers.extend_from_within(..2);
+    /// assert_eq!(numbers, [0, 1, 2, 3, 4, 0, 1]);
     ///
-    /// vec.extend_from_within(4..8);
-    /// assert_eq!(vec, [0, 1, 2, 3, 4, 2, 3, 4, 0, 1, 4, 2, 3, 4]);
+    /// let mut strings = vec![String::from("hello"), String::from("world"), String::from("!")];
+    /// strings.extend_from_within(1..=2);
+    /// assert_eq!(strings, ["hello", "world", "!", "world", "!"]);
     /// ```
     #[cfg(not(no_global_oom_handling))]
     #[stable(feature = "vec_extend_from_within", since = "1.53.0")]