Remove references to Map.maxExtent

I have searched for map.maxExtent and, in Map.js, for this.maxExtent, and after this change the only remaining reference to map.maxExtent should be as the default value in Layer.js

I have clarified the docs in Map and Layer, and added a release note as suggested in #825

As projection/units/maxExtent/minExtent in Map are now only used as default for layer constructor, It might be better if they were no longer APIProperties but documented as constructor options.