zeroize/derive: add docs for the Zeroize proc macro (#504)

Author: tony-iqlusion

zeroize/derive/src/lib.rs

@@ -14,6 +14,18 @@ use quote::quote;
 use syn::{Attribute, Meta, NestedMeta};
 use synstructure::{decl_derive, BindStyle};
 
+decl_derive!(
+    [Zeroize, attributes(zeroize)] =>
+
+    /// Derive the `Zeroize` trait.
+    ///
+    /// Supports the following attribute:
+    ///
+    /// - `#[zeroize(drop)]`: derives the `Drop` trait, calling `zeroize()`
+    ///   when this item is dropped.
+    derive_zeroize
+);
+
 /// Name of zeroize-related attributes
 const ZEROIZE_ATTR: &str = "zeroize";
 
@@ -30,8 +42,6 @@ fn derive_zeroize(s: synstructure::Structure<'_>) -> TokenStream {
     }
 }
 
-decl_derive!([Zeroize, attributes(zeroize)] => derive_zeroize);
-
 /// Custom derive attributes for `Zeroize`
 #[derive(Default)]
 struct ZeroizeAttrs {