Add doc comments for Unwrap, IsVariant

[Technohacker]

Prevents a linting issue from missing_docs on the derived functions

Fixes #159

[JelteF]

It seems the new code does not compile on 1.36 because of this issue rust-lang/rust#52607
The second comment in the issue describes a workaround that can probably be used. This bug was only fixed in 1.50

Can you check if it's possible to use this workaround?

[Technohacker]

Hmm, I'll have a check on that

[Technohacker]

As an aside, would it make sense to add something like unwrap_*_ref and unwrap_*_ref_mut functions to the Unwrap derive, or maybe make a new derive macro for these? Intended for something similar to the enum_as_inner crate, but without having to unwrap the options in between when the type of enum is known

[Technohacker]

Turns out my previous code would have expanded the stringify! macro after the Unwrap/IsVariant derive macros, rather than expanding it within the derive itself. New change should fix that