Correct add-on placement logic documentation
Correct add-on placement logic documentation

- we previously documented the add-on rules as if they were OR'ed
together. This is incorrect as the top level rules are AND'ed together
and only the items in individual rules are OR'ed together.
- this results in confusion and a bug report

[#166749915](https://www.pivotaltracker.com/story/show/166749915)

Co-authored-by: Rebecca Putinski <1769705669056ac7df0c2658a01b1e2790d3ae82@pivotal.io>

cloudfoundry/docs-bosh