Add documentation for inter-pod affinity/anti-affinity

[davidopp]

I'm not sure why the code include lines aren't working; it seems to be following the directions in
http://kubernetes.io/editdocs/
for how to include text from another file.

@kubernetes/docs to review and hopefully answer the question above

cc/ @kevin-wangzefeng just FYI

---

This change is

[davidopp]

TODO: make this and the next line start with two dashes, not one

[janetkuo]

=== RUN   TestExampleObjectSchemas
--- FAIL: TestExampleObjectSchemas (0.03s)
    examples_test.go:293: ../docs/user-guide/node-selection/pod-with-pod-affinity.yaml: pod-with-pod-affinity does not have a test case defined

Add docs/user-guide/node-selection/pod-with-pod-affinity.yaml to the example tests in examples_test.go

[janetkuo]

@kevin-wangzefeng wanna take a look?

[davidopp]

I screwed up this PR pretty badly in my local client so have replaced it with #1148.