Rollup merge of #72986 - pickfire:vec-assert, r=Mark-Simulacrum

Add more assert to Vec with_capacity docs

Show assertion on len too to show them how adding new items will affect both the
length and capacity, before and after.

src/liballoc/vec.rs

@@ -343,15 +343,18 @@ impl<T> Vec<T> {
     ///
     /// // The vector contains no items, even though it has capacity for more
     /// assert_eq!(vec.len(), 0);
+    /// assert_eq!(vec.capacity(), 10);
     ///
     /// // These are all done without reallocating...
     /// for i in 0..10 {
     ///     vec.push(i);
     /// }
+    /// assert_eq!(vec.len(), 10);
     /// assert_eq!(vec.capacity(), 10);
     ///
     /// // ...but this may make the vector reallocate
     /// vec.push(11);
+    /// assert_eq!(vec.len(), 11);
     /// assert!(vec.capacity() >= 11);
     /// ```
     #[inline]