Make use of 'work group' consistent across ARB/EXT specs

Fixes https://www.khronos.org/members/login/bugzilla/show_bug.cgi?id=11723 for
ARB / EXT / OES specs (already fixed in core API specs).

N.b. there appear to be many remaining instances of inconsistent use of
'work group' in NV extension specs, which they can fix if they want to.

There are some incidental whitespace diffs.
7 files changed