modlua: Removing unhelpful proxy function to dispatching lua module hooks.

This cleanup is a prelude to adding an extra argument, the hook data
element, to the lua callback.
modlua lua modules: Make hook data available to Lua callbacks.
modlua lua modules: Documentation for hook callback data available to Lua modules.
docs: Reduce details in preface.
docs: Add description.
docs: Cleanup the introduction and add in links to section references.
docs: Add missing actions and cleanup text and syntax.