Use "annotation" for the return_annotation param description

The param holds an inspect-style annotation (it accepts the
inspect.Parameter.empty sentinel, which is not a type hint) and is
named return_annotation. "Type hint" stays in the caveat and the
args/kwargs dict-form descriptions, where the claim is genuinely about
expressing types.

Author: hmgaudecker

src/dags/signature.py

@@ -110,7 +110,7 @@ def with_signature(
         enforce: Whether the signature should be enforced or just
             added to the function for introspection. This creates runtime
             overhead.
-        return_annotation: The return type hint. By default, the return annotation is
+        return_annotation: The return annotation. By default, it is
             `inspect.Parameter.empty`.
 
     Returns: