Make the Swift-related doc more explicit

Thanks to Yassine Lamgarchal for noticing the documentation did not explicitely
state where 'ceilometer' must be found in the pipeline.

While we're at it, fix a typo: "these lines" should be used instead of "this
lines".

Change-Id: Iffff9b779d48bafefb249bba0f4db4a006f94a70
Merge "Make the Swift-related doc more explicit"