Clarify documentation as per:
Clarify documentation as per:

- https://stackoverflow.com/questions/46952357/dummy-sampler-under-runtime-controller-duplicate-requests

git-svn-id: https://svn.apache.org/repos/asf/jmeter/trunk@1813452 13f79535-47bb-0310-9956-ffa450edef68

Former-commit-id: 104b8912e293b2a1ab8aa0f31cd99eb5e49894c9
apache/jmeter,apache/jmeter,benbenw/jmeter,apache/jmeter,benbenw/jmeter,ham1/jmeter,etnetera/jmeter,etnetera/jmeter,etnetera/jmeter,ham1/jmeter,apache/jmeter,benbenw/jmeter,apache/jmeter,ham1/jmeter,benbenw/jmeter,etnetera/jmeter,ham1/jmeter,etnetera/jmeter,ham1/jmeter