Fix variant name interpolation in documentation for IsVariant (#360)

Resolves #357 

## Synopsis

Generated documentation is incorrect, see issue

## Solution

Ensure `stringify!` happens after `quote!` interpolation

CHANGELOG.md

@@ -99,6 +99,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
  ([#297](https://github.com/JelteF/derive_more/pull/297))
 - Hygiene of macro expansions in presence of custom `core` crate.
  ([#327](https://github.com/JelteF/derive_more/pull/327))
+- Fix documentation of generated methods in `IsVariant` derive.
 
 ## 0.99.10 - 2020-09-11
 

impl/src/is_variant.rs

@@ -39,10 +39,9 @@ pub fn expand(input: &DeriveInput, trait_name: &'static str) -> Result<TokenStre
  Fields::Unnamed(_) => quote! { (..) },
  Fields::Unit => quote! {},
  };
- let variant_name = stringify!(variant_ident);
  let func = quote! {
  #[doc = "Returns `true` if this value is of type `"]
- #[doc = #variant_name]
+ #[doc = stringify!(#variant_ident)]
  #[doc = "`. Returns `false` otherwise"]
  #[inline]
  #[must_use]