Add Vec visualization to understand capacity

Visualize vector while differentiating between stack and heap.

Inspired by cheats.rs, as this is probably the first place beginner go,
they could understand stack and heap, length and capacity with this. Not
sure if adding this means we should add to other places too.

Superseeds rust-lang#76066

Author: pickfire

alloc/src/vec/mod.rs

@@ -253,6 +253,26 @@ mod spec_extend;
 /// can be slow. For this reason, it is recommended to use [`Vec::with_capacity`]
 /// whenever possible to specify how big the vector is expected to get.
 ///
+/// A vector containing the elements `'a'` and `'b'` with capacity 4 can be
+/// visualized as:
+///
+/// ```text
+/// Stack       ptr      len  capacity
+/// /Heap  +--------+--------+--------+
+///        | 0x0123 |      2 |      4 |
+///        +--------+--------+--------+
+///             |
+///             v
+/// Heap   +--------+--------+--------+--------+
+///        |    'a' |    'b' | uninit | uninit |
+///        +--------+--------+--------+--------+
+/// ```
+///
+/// - **uninit** represents memory that is not initialized, see [`MaybeUninit`].
+/// - Note: the ABI is not stable and `Vec` makes no guarantees about its memory 
+///   layout (including the order of fields). See [the section about 
+///   guarantees](#guarantees).
+///
 /// # Guarantees
 ///
 /// Due to its incredibly fundamental nature, `Vec` makes a lot of guarantees
@@ -345,6 +365,7 @@ mod spec_extend;
 /// [`push`]: Vec::push
 /// [`insert`]: Vec::insert
 /// [`reserve`]: Vec::reserve
+/// [`MaybeUninit`]: core::mem::MaybeUninit
 /// [owned slice]: Box
 /// [slice]: ../../std/primitive.slice.html
 /// [`&`]: ../../std/primitive.reference.html