one-line-per-sentence policy for the section to update

* to make it easier to see the differences
clarify a bit the difference between master- and slave-side source steps

* emphasise the difference between them
* since master-side steps are recommended move them up in the examples
* slave-steps are deprecated and will be removed in 0.9
* emphasise that the next section is about master-side steps
Merge sa2ajj/buildbot:new-old-source-steps (PR #1252)