Update intrinsics documentation

documentation/intrinsics.md

@@ -17,6 +17,14 @@ Simply returns the size of type `T` in bits.
 i32::sizeof(); // Returns 32
 ```
 
+#### `<T>::null() -> *T`
+
+Returns a null-pointer of type `T`.
+
+```rust
+i32::null(); // Returns *i32 (null-ptr)
+```
+
 #### `<T>::alloca(size: u64) -> *T`
 
 Allocates `T::sizeof() * size` bits and returns a pointer to `T`.