Crossref'd transfer-limit and handoff-concurrency

In response to issue #758, the following commit creates a cross
reference between the `riak-admin transfer-limit` and the
`handoff_concurrency` setting of the `riak_core` section of `app.config`
Merge pull request #760 from angrycub/enhancement/cv/handoff_concurrency_xref

Crossref'd transfer-limit and handoff-concurrency