Improve docs for ol.proj functions

I've left Proj4js for the moment, until it's decided what to do with this. Once that's done, there should probably be a separate tutorial on the whole subject of projections/transforms. In the meantime, this PR should make things a bit clearer.
I'm not sure that its better to describe *how* a function does something as opposed to *what* the function does.  Referencing the other methods that this method calls to do its work seems like unnecessary and potentially confusing detail.