Bring back reference doc for built-in GLSL functions/uniforms/constants/etc

Just for those useful to post-processing.

Let's use `@private` (if it doesn't just work out of the box already) for the others.

Locate the previous reference documentation for built-in GLSL functions, uniforms, and constants, then inspect the current documentation entry point and visibility configuration. Confirm which items are intended for post-processing and verify that the remaining built-ins are private; done means the reference is available again with the requested visibility.

Written by the indexing model from the issue text.