Update dev_guide.services.$location.ngdoc

Added a point about using Angular in the root of a domain with HTML5 urls and then trying to link to external paths within the same directory.

Please see the following:
Original issue: https://github.com/angular/angular.js/issues/3520#issuecomment-22393487
Solution: http://fiddle.jshell.net/fgHf6/3/