docs(decorators): add info about async createParamDecorator

Author: MichalLytek

docs/custom-decorators.md

@@ -52,15 +52,19 @@ Or might be a more advanced one that performs some calculations and encapsulates
 
 ```typescript
 function Fields(level = 1): ParameterDecorator {
-  return createParamDecorator(({ info }) => {
+  return createParamDecorator(async ({ info }) => {
     const fieldsMap: FieldsMap = {};
     // calculate an object with info about requested fields
     // based on GraphQL `info` parameter of the resolver and the level parameter
+
+    // or even call some async service, as it can be a regular async function and we can just `await`
     return fieldsMap;
   });
 }
 ```
 
+> Be aware, that async function as a custom param decorators logic can make the GraphQL resolver execution slower, so try to avoid them, if possible.
+
 Then we can use our custom param decorators in the resolvers just like the built-in decorators:
 
 ```typescript