[0004] Update union spec to clarify casting behaviors

[llvm-beanz]

Which proposal does this relate to?
0004-unions

Describe the issue or outstanding question.
As per the discussion in #37, C++ union rules actually might not really be what we want for HLSL.

The C++ standard states the following in §9.5.1:

In a union, at most one of the non-static data members can be active at any time, that is, the value of at most one of the non-static data members can be stored in a union at any time. [...] The size of a union is sufficient to contain the largest of its non-static data members. Each non-static data member is allocated as if it were the sole member of a struct. All non-static data members of a union object have the same address.

We should elaborate the unions spec to clarify some of these behaviors one way or the other.

Additional context
vk::BufferPointer<T,A> review
Related vk::BufferPointer<T,A> issue