Change "director-level scopes" to "top-level scopes", add sentence about scopes
Change "director-level scopes" to "top-level scopes", add sentence about scopes

[#161020991](https://www.pivotaltracker.com/story/show/161020991)

Co-authored-by: Denise Yu <d2c4da7d6ea264fda124d8d8d8780e90b8d0b85a@pivotal.io>

cloudfoundry/docs-bosh