[SYCL][Doc] Update sub-group docs#1565

Merged

bader merged 6 commits intointel:syclfrom

Pennycook:sub-group-docs-update

Jun 19, 2020

Commits on Apr 21, 2020

Commits on Apr 22, 2020

[SYCL][Doc] Align sub-groups with other extensions
Pennycook
committed

Commits on Apr 28, 2020

[SYCL][Doc] Clarify usage of two get_info calls
Pennycook
committed

Commits on Jun 18, 2020

[SYCL] Remove fence_space from sub-group barrier
Pennycook
committed