Merge pull request #3956 from Tanguille/docs/clarify-as-expression-to…

…-sql

docs: add mention of requirement to implement `ToSql`

diesel_derives/src/lib.rs

@@ -134,6 +134,8 @@ pub fn derive_as_changeset(input: TokenStream) -> TokenStream {
 /// you can specify this by adding the annotation `#[diesel(not_sized)]`
 /// as attribute on the type. This will skip the impls for non-reference types.
 ///
+/// Using this derive requires implementing the `ToSql` trait for your type.
+///
 /// # Attributes:
 ///
 /// ## Required container attributes