Clarify the difference between AnyLuaValue values and the specialised Lua* structs

[linkmauve]

It’s unclear in the documentation which should be used in which case.

Another option would be to unify them, I guess.

[tomaka]

If you read a specialized Lua* struct, you will get either a conversion (eg. you're trying to read a number into a string) or an error (eg. you're trying to read a function into a string).