Refine docs and comments etc.

src/fn_func.rs

@@ -12,6 +12,7 @@ pub trait Func<ARGS, RET> {
     type Output;
 
     /// Create a Rust closure from an [`AST`].
+    ///
     /// The [`Engine`] and [`AST`] are consumed and basically embedded into the closure.
     ///
     /// # Example
@@ -43,6 +44,7 @@ pub trait Func<ARGS, RET> {
     fn create_from_ast(self, ast: AST, entry_point: &str) -> Self::Output;
 
     /// Create a Rust closure from a script.
+    ///
     /// The [`Engine`] is consumed and basically embedded into the closure.
     ///
     /// # Example