Permissive handling for properties with dots.

With can-map 3.2.0, handling of dotted properties (e.g. "foo.bar" as a property key in a can-map) was updated to throw fewer errors and allow more cases, but this was not documented as official behavior in the API. In this release, we've made those changes official in the API.

For more information, see the "Properties with dots in their name" section in https://github.com/canjs/can-map/blob/v3.4.0/docs/prototype.attr.md