docs(compile): document $attrs.$normalize

In order to keep the "terminology" consistent with the rest of the docs, I would rephraze it to something like:

    Converts an attribute name (e.g. dash/colon/underscore-delimited string, optionally prefixed with `x-` or `data-`) to its normalized, camelCase form.

You could also add a link to https://docs.angularjs.org/guide/directive#matching-directives for further info.