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.
Ah, I see, its not a typo its a list.  Can we use a markdown list here instead?  Or a) and b) - using (a) is confusing I think