Copied spec text for clientWaitSync from OpenGL ES 3.0 spec. Per WebGL F2F, mandated that implementations enforce a low but unspecified maximum timeout. Stated that applications should not use clientWaitSync to block execution.
Merge pull request #710 from kenrussell/client-wait-sync

Copied spec text for clientWaitSync from OpenGL ES 3.0 spec. Per WebGL F...