[Buildbot] #2929: document BuildStep renderables variable

[Buildbot trac]

#2929: document BuildStep renderables variable
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Reporter:  vlovich  |      Owner:
    Type:  task     |     Status:  new
Priority:  major    |  Milestone:  undecided
 Version:  0.8.9    |   Keywords:
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 Documentation for the 'renderables' property is missing which is needed
 for writing custom steps.  Probably add a description in the buildbot
 customization section?

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Ticket URL: <http://trac.buildbot.net/ticket/2929>
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Buildbot: build/test automation