Rollup merge of #113147 - lizhanhui:fix_vec_from_raw_parts_doc_example, r=Mark-Simulacrum

Fix document examples of Vec::from_raw_parts and Vec::from_raw_parts_in

These two examples are misplaced.

library/alloc/src/vec/mod.rs

@@ -560,22 +560,20 @@ pub fn with_capacity(capacity: usize) -> Self {
     /// Using memory that was allocated elsewhere:
     ///
     /// ```rust
-    /// #![feature(allocator_api)]
-    ///
-    /// use std::alloc::{AllocError, Allocator, Global, Layout};
+    /// use std::alloc::{alloc, Layout};
     ///
     /// fn main() {
     ///     let layout = Layout::array::<u32>(16).expect("overflow cannot happen");
     ///
     ///     let vec = unsafe {
-    ///         let mem = match Global.allocate(layout) {
-    ///             Ok(mem) => mem.cast::<u32>().as_ptr(),
-    ///             Err(AllocError) => return,
-    ///         };
+    ///         let mem = alloc(layout).cast::<u32>();
+    ///         if mem.is_null() {
+    ///             return;
+    ///         }
     ///
     ///         mem.write(1_000_000);
     ///
-    ///         Vec::from_raw_parts_in(mem, 1, 16, Global)
+    ///         Vec::from_raw_parts(mem, 1, 16)
     ///     };
     ///
     ///     assert_eq!(vec, &[1_000_000]);
@@ -758,19 +756,22 @@ pub fn with_capacity_in(capacity: usize, alloc: A) -> Self {
     /// Using memory that was allocated elsewhere:
     ///
     /// ```rust
-    /// use std::alloc::{alloc, Layout};
+    /// #![feature(allocator_api)]
+    ///
+    /// use std::alloc::{AllocError, Allocator, Global, Layout};
     ///
     /// fn main() {
     ///     let layout = Layout::array::<u32>(16).expect("overflow cannot happen");
+    ///
     ///     let vec = unsafe {
-    ///         let mem = alloc(layout).cast::<u32>();
-    ///         if mem.is_null() {
-    ///             return;
-    ///         }
+    ///         let mem = match Global.allocate(layout) {
+    ///             Ok(mem) => mem.cast::<u32>().as_ptr(),
+    ///             Err(AllocError) => return,
+    ///         };
     ///
     ///         mem.write(1_000_000);
     ///
-    ///         Vec::from_raw_parts(mem, 1, 16)
+    ///         Vec::from_raw_parts_in(mem, 1, 16, Global)
     ///     };
     ///
     ///     assert_eq!(vec, &[1_000_000]);