Add information on range checking and primitive restart in WebGL 2

Includes filling out some differences between WebGL 1 and WebGL 2.
Typo fixes
WebGL2: IsEnabled docs and formatting for "Setting and getting state"

RASTERIZER_DISCARD is missing from the GLES3 man pages, but is in the
spec, so add that to the list of queriable parameters. Add spec link for
getParameter.
Merge pull request #627 from NVIDIA/primitive-restart-range-check

Add information on range checking and primitive restart in WebGL 2