A few comments:
  * are you following some convention about the spelling of buildbot/Buildbot/BuildBot?
  * "+3. the standard development toolchain."
    Which is? I think a compiler is the only missing bit.
  * "(in which case you'll have to install Python and Twisted first)."
    I would say "you'll need" and not assume that they are not installed.
  * "a couple of third-party libraries (such as zlib and OpenSSL)."
    Actually these are not required, and it might even be good to have some buildbot without e.g. zlib.
  * "+   % su - buildbot"
    I think we use $ elsewhere in the doc.  You can also set the highlightlang to sh (iirc).
  * "/usr/local/bin", these should get some markup: either ``..`` or :file:`...` (is :file: ok for dirs too?).
  * "You'll need to get someone to create the slavename/slavepasswd on dinsdale.python.org before doing this."
    So maybe put it before?
  * "monitor the build results and solve any setup issues causing test failures"
    If running a buildbot implies some responsibilities, maybe they should be mentioned earlier.
  * "executing arbitrarily horrible and/or malicious code"
    This sounds bad and misleading.  At worst the test can use resources extensively and/or cause crashes, but that doesn't mean that the code is arbitrary, horrible, or malicious.
  * "for cutting of the builder"
    s/of/off/
  * "Therefore builders should be set up in both configurations"
    This might lead build masters to think they have to set up both.
  * "Some tests, as the start of `this thread "
    You already have a link to this at the end of the section
  * "+Running a build slave requires some network access."
    Isn't 9020 the only *required* one?  If the others are not available, test will (should) be skipped.