Include the CannotParseBodyAsJSON behaviour in json's docstring

src/requtests/parsed_request.py

@@ -35,6 +35,8 @@ def headers(self) -> Dict[str, str]:
     def json(self) -> Any:
         """
         The body of the prepared request, parsed as JSON.
+
+        Raises a CannotParseBodyAsJSON error if the body is not valid JSON.
         """
         try:
             return json.loads(self.prepared_request.body)