Document a list of "reserved" variables in hook_theme that are already used by drupal_render and the FAPI

There are some variables that should just not be declared in hook_theme() or they run the risk of conflicting with attributes used by drupal_render() and/or the FAPI.

Examples:

This should all be clearly documented in the docblock for hook_theme as many themers would be unaware of all the items in the full list.