Update docs

- Accurately label the type of fields in FromVariant derivers
- Add bounds constraint to magic field docs

Fix #200

src/lib.rs

@@ -61,7 +61,7 @@
 //! |---|---|---|
 //! |`ident`|`syn::Ident`|The identifier of the passed-in variant|
 //! |`discriminant`|`Option<syn::Expr>`|For a variant such as `Example = 2`, the `2`|
-//! |`fields`|`Option<darling::ast::Fields<__>>`|The fields associated with the variant|
+//! |`fields`|`darling::ast::Fields<T> where T: FromField`|The fields associated with the variant|
 //! |`attrs`|`Vec<syn::Attribute>`|The forwarded attributes from the passed in variant. These are controlled using the `forward_attrs` attribute.|
 
 extern crate core;