Document command-line flags; other doc tweaks
Document command-line flags; other doc tweaks

git-svn-id: https://svn.apache.org/repos/asf/jakarta/jmeter/branches/rel-2-1@326766 13f79535-47bb-0310-9956-ffa450edef68

Former-commit-id: 00e5ace5c7680b7c94210ade6e83a476c758d08c
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