Clarify helper behavior to match Vulkan spec

[Tobski]

Originally there was a "if they are active then they participate" clause in the Vulkan spec which was confusing, so we updated it to the current language here: https://www.khronos.org/registry/vulkan/specs/1.3/html/chap9.html#shaders-helper-invocations

This change brings the GLSL extension in line with the API spec

[Tobski]

@gnl21 can this be merged? Just going back through my outstanding todo list and found this dangling.

[gnl21]

Sorry, done.