Loop layer: document pattern for asynchronous services

Asynchronous services in T1 need to be polled using a nested workflow containing the 'fail if false' local worker in order to execute them. This will not work in T2, so we need to specify the despatch stack pattern to replicate this behaviour when translating T1 workflows or when creating new T2 workflows with asynchronous services

Solution:
Add documentation on how to use the Loop layer to use asynchronous services.

Test:
Test the documentation on a real user together with an asynchronous EBI service.