[Buildbot] #3465: buildbot.steps.worker.CompositeStepMixin is not documented

[Buildbot trac]

#3465: buildbot.steps.worker.CompositeStepMixin is not documented
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Reporter:  rutsky     |      Owner:
    Type:  undecided  |     Status:  new
Priority:  major      |  Milestone:  undecided
 Version:  0.9.0b7    |   Keywords:
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 Is this class is part of public API?
 It's not exported as 'steps' plugin in `setup.py`.

 If it's deprecated/shouldn't be used, lets add explicit
 DeprecationWarning.

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Ticket URL: <http://trac.buildbot.net/ticket/3465>
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