[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);

[Naghasan]

out of curiosity, why removing the possibility to select the yes/no decoration ?

[Pennycook]

The logic was that there shouldn't be two ways to do the same thing, because that's harder to teach.

Before this change, *_address_cast functions could be used to cast between address spaces and to introduce decoration. Now, they can only be used to cast between address spaces, which is aligned with their name.

multi_ptr already provides interfaces to move between decorated and undecorated pointers (via implicit casts), so we don't need to handle it here as well.

[Pennycook]

@intel/llvm-gatekeepers I think this can be merged now.