ng-xi18n -p is not documented

As per angular/angular-cli#2201 (comment)
This command is not documented on the Angular site: https://angular.io/docs/ts/latest/cookbook/i18n.html

[vicb]

similar to tsc -p or ngc -p (ie path to the tsconfig)

sure, but it should be documented somewhere, right? :)

[wardbell]

OK. Coming in PR #2740