[HLSL] Document HLSL buffer types in the language spec

[damyanp]

We need to document the set of buffer types in the language spec. This should list the following types as available, document constraints on the template parameters, and list the valid operations.

template <typename T> class Buffer;
template <typename T> class RWBuffer;
template <typename T> class StructuredBuffer;
template <typename T> class RWStructuredBuffer;
class ByteAddressBuffer;
class RWByteAddressBuffer;