sycl namespace

The reference assumes `using namespace sycl;` in synopses and text.

@keryell suggested that instead namespaces should be explicit everywhere. Should we make them explicit in both the code snips and all the text within SYCL_Reference, or only in the code snips?

Currently we're mixing styles. For example:



Review the SYCL_Reference code snippets and surrounding text to inventory the mixed namespace styles shown in the issue. First resolve whether explicit namespaces apply to snippets only or to all prose, then update the affected reference content consistently; done means the chosen convention is applied throughout.

Written by the indexing model from the issue text.