finagle, scrooge: Improve docs on ServiceIface and FutureIface
finagle, scrooge: Improve docs on ServiceIface and FutureIface

Summary: Problem

The documentation for Scrooge's ServiceIface and FutureIface/MethodIface
was lacking and in some cases confusing.

Solution

Tweak the examples and documentation to add clarity.

JIRA Issues: csl-4680

Differential Revision: https://phabricator.twitter.biz/D67345

mkhq/finagle,luciferous/finagle,koshelev/finagle,luciferous/finagle,mkhq/finagle,twitter/finagle,twitter/finagle,twitter/finagle,luciferous/finagle,twitter/finagle,koshelev/finagle,luciferous/finagle,mkhq/finagle,twitter/finagle,koshelev/finagle,luciferous/finagle,koshelev/finagle,koshelev/finagle,luciferous/finagle,mkhq/finagle,mkhq/finagle,mkhq/finagle,koshelev/finagle