[Buildbot] #3097: document waited_for arg to addBuildset methods

[Buildbot trac]

#3097: document waited_for arg to addBuildset methods
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Reporter:  dustin       |      Owner:  dustin
    Type:  enhancement  |     Status:  new
Priority:  major        |  Milestone:  0.9.0
 Version:               |   Keywords:
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 See
 https://github.com/buildbot/buildbot/commit/72371252ef208da1480b436cb18a579543174b64#commitcomment-8866109

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Ticket URL: <http://trac.buildbot.net/ticket/3097>
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