Mention alloc too

src/std.md

@@ -22,9 +22,10 @@ The common vocabulary types include:
 
 <details>
   
-  * In fact, Rust contains two layers of the Standard Library: `core` and `std`. 
+  * In fact, Rust contains several layers of the Standard Library: `core`, `alloc` and `std`. 
   * `core` includes the most basic types and functions that don't depend on `libc`, allocator or
-    even a presense of the operating system. 
-  * Embedded Rust applications often only use `core`.
+    even the presence of an operating system. 
+  * `alloc` includes types which require a global heap allocator, such as `Vec`, `Box` and `Arc`.
+  * Embedded Rust applications often only use `core`, and sometimes `alloc`.
 
 </details>