finagle: Add docs for `c.t.f.offload.auto` and `c.t.f.offload.admissionControl` flags
finagle: Add docs for `c.t.f.offload.auto` and `c.t.f.offload.admissionControl` flags

Problem

We have these two experimental flags which are pretty powerful for
improving service performance but they're not even documented.

Solution

Document them.

Differential Revision: https://phabricator.twitter.biz/D849630

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