added explanation for the columns in the aggregate report.
added explanation for the columns in the aggregate report.

peter

git-svn-id: https://svn.apache.org/repos/asf/jakarta/jmeter/branches/rel-2-1@396574 13f79535-47bb-0310-9956-ffa450edef68

Former-commit-id: 6f0d22fdcb14792c5d9afd22162bcc8fdb57c545
ham1/jmeter,apache/jmeter,etnetera/jmeter,benbenw/jmeter,benbenw/jmeter,benbenw/jmeter,etnetera/jmeter,apache/jmeter,apache/jmeter,etnetera/jmeter,etnetera/jmeter,ham1/jmeter,ham1/jmeter,ham1/jmeter,apache/jmeter,benbenw/jmeter,etnetera/jmeter,ham1/jmeter,apache/jmeter