scylla: Add empty lines in old derive macros docs

Without those lines, whole doc for an item is merged and shown
in the summary, which makes it hard to read.

scylla/src/macros.rs

@@ -1,18 +1,21 @@
 /// #[derive(FromRow)] derives FromRow for struct
+///
 /// Works only on simple structs without generics etc
 ///
 /// ---
 ///
 pub use scylla_cql::macros::FromRow;
 
 /// #[derive(FromUserType)] allows to parse struct as a User Defined Type
+///
 /// Works only on simple structs without generics etc
 ///
 /// ---
 ///
 pub use scylla_cql::macros::FromUserType;
 
 /// #[derive(IntoUserType)] allows to pass struct a User Defined Type Value in queries
+///
 /// Works only on simple structs without generics etc
 ///
 /// ---