docs(CONTRIBUTING.md): explain we cannot accept localization changes

Angular uses the I18N code from Closure library to generate its own localization
files. So there is no point submitting pull requests for these files, since
all changes would be lost when we next generate these files.

Closes #4610
docs(ngdoc.js): fix usage to display attributes as dash-case

Closes 4611
docs(ngForm): ensure correct attribute name in usage example

Closes #4611