finagle/doc: MethodBuilder docs for idempotency
finagle/doc: MethodBuilder docs for idempotency

Summary: Problem / Solution

The MethodBuilder docs are missing a section for idempotency.
Add it, and modify the example to use idempotent/nonIdempotent
methods.

JIRA Issues: CSL-5774

Differential Revision: https://phabricator.twitter.biz/D128417

luciferous/finagle,luciferous/finagle,twitter/finagle,luciferous/finagle,twitter/finagle,twitter/finagle,luciferous/finagle,luciferous/finagle,twitter/finagle,twitter/finagle,luciferous/finagle