Improve documentation about policy execution from tasks

The documentation doesn't explain the logic of tasks execution:

• if task URL and METHOD are set, but requestPolicy is not set, the task will execute an HTTP request to the given URL
• if task URL and METHOD are set, and requestPolicy is also set, the task will execute the request policy and will ignore the URL and METHOD
• if task URL and METHOD are not set, and requestPolicy is set, the task will execute the request policy
• if neither URL and METHOD nor requestPolicy is set, the task cannot be executed, because it can't do anything.

In a nutshell - tasks either execute a requestPolicy OR make HTTP request to a given URL - not both.