docs(directive): Clarified and cleaned up directive guide

- corrected terminology about how directives use `require`
- added more variations to the DirectiveDefinitionObject
- removed some slightly superfluous text

docs(directive): Minor correction to example to avoid bad practice

Anchor tags should use `ng-href` instead of `href` for interpolation.

docs(directive): Supplementing DDO description

DDO = Directive Definition Object
Tweak recommended here:
https://github.com/angular/angular.js/pull/2888/files#r4664565