Clarify user data in 4.1

["Curt Carpenter" <curtc@...>]

So what's the best way in 4.1 to get a void* from C to Lua, and back,
since lua_pushusertag and lua_pushuserdata are not in the tags
compatibility layer?

Sorry for the probable redundancy. I reviewed recent posts but didn't
see an answer.

Thanks,

Curt