Add missing documentation about 'Same user on each iteration' for Thread Groups.
Add missing documentation about 'Same user on each iteration' for Thread Groups.

Re-added this bit, as it seems to got lost somehow.

Originally contributed by njkuzas with commit acdb6482dd3e945e0e8acefd7109b2df3faf505c

Part of #643
Bugzilla Id: 65128

apache/jmeter,ham1/jmeter,ham1/jmeter,apache/jmeter,ham1/jmeter,apache/jmeter,ham1/jmeter,apache/jmeter,apache/jmeter,ham1/jmeter