[SYCL][Doc] Update address_cast specification

[Pennycook]

This PR contains three improvements to the specification:

• Default address_cast to no decoration.
• Add overloads for generic multi_ptr.
• Add address_space shorthands.

This allows developers using generic raw pointers to cast to multi_ptr without worrying about decoration:

float* ptr = ...;
auto mptr = syclex::static_address_cast<syclex::local_space>(ptr);

...or to opt-in to decoration by using a generic multi-pointer, which propagates decoration information:

multi_ptr<float, syclex::generic_space, access::decorated::yes> decorated_in = ...;
auto decorated_out = syclex::static_address_cast<syclex::local_space>(decorated_in);