Add warning for #[repr(C)] struct alignment pseudocode

Author: kinseytamsin

src/type-layout.md

@@ -226,6 +226,15 @@ for field in struct.fields_in_declaration_order() {
 struct.size = current_offset + padding_needed_for(current_offset, struct.alignment);
 ```
 
+<div class="warning">
+
+Warning: The function used to calculate the needed padding in this pseudocode is
+a naive implementation that doesn't account for overflow issues. For a rigorous
+implementation suitable for performing this calculation in actual code, see the
+source code for [`Layout::padding_needed_for`].
+
+</div>
+
 > Note: This algorithm can produce zero-sized structs. This differs from
 > C where structs without data still have a size of one byte.
 
@@ -374,3 +383,4 @@ used with any other representation.
 [`C`]: #the-c-representation
 [primitive representations]: #primitive-representations
 [`transparent`]: #the-transparent-representation
+[`Layout::padding_needed_for`]: ../std/alloc/struct.Layout.html#method.padding_needed_for