Add type punning rules to vk::BufferPointer (#55)

* Add type punning rules to vk::BufferPointer

Fixes #43

* Clarify that type punning with buffer pointers leads to undefined
behavior.

proposals/0010-vk-buffer-ref.md

@@ -175,6 +175,10 @@ A vk::BufferPointer can otherwise be used whereever the HLSL spec does not other
 
 Applying HLSL semantic annotations to objects of type vk::BufferPointer is disallowed.
 
+### Buffer Pointers and Type Punning Through Unions
+
+While buffer pointer types are allowed in unions, type punning with buffer pointer types is disallowed as it is with all other types in HLSL. Specifically, when a member of a union is defined, all other members become undefined, no matter the types.
+
 ## SPIR-V Appendices
 
 ### Appendix A: SPIR-V for RawBufferLoad