[SYCL][Doc] Modernize LocalMemory extension

[Pennycook]

Brings LocalMemory extension in line with other extensions:

• Written against SYCL 2020
• Feature test macro
• sycl::ext::oneapi namespace

This commit replaces the old SYCL 1.2.1 extension completely
in favor of a SYCL 2020 extension, because the SYCL 1.2.1 extension
has not yet been marked implemented.

Signed-off-by: John Pennycook [email protected]

[Pennycook]

Modernizing the documentation ahead of marking the extension "implemented" as discussed in #4323. While converting to the new extension format I noticed a few typos and copy-paste errors, which this commit also fixes.

@sergey-semenov: Tagging you so that you can double-check this matches the implementation.

[oleksandr-pavlyk]

The example is most useful. It would be further useful if the document better explained how group_local_memory<int[64]>(item.get_group()) is different from group_local_memory_for_overwrite<int[64]>(item.get_group());

[gmlueck]

NOTE: This document is better viewed when rendered as html with asciidoctor. GitHub does not render image icons.

I did not include this statement in some of the more recent spec's I've written. In my experience GitHub renders the asciidoctor sources pretty well, and I think we would want to encourage people to use the GitHub rendering. Asking people to clone the repo, install asciidoctor, and run it manually is a real pain. Do you really think we need this statement?

Built On: {docdate} +

I also did not include this line in my recent specs. When you use the GitHub rendering, this simply shows the current date, which is not useful. If you want the spec to contain a date, it would be better to hard-code here. Alternatively, it may be better to use use the revision number to identify revisions of the spec.