Document pending Serving pod spec properties.

[skonto]

What is changing? (Please include as many details as possible.)

New pod spec properties are available: hostpid, hostnetwork, hostipc, shareprocessnamespace.

How will this impact our users?

In what release will this take happen (to the best of your knowledge)?

Ex. v0.2-v0.3

Context

Link to associated PRs or issues from other repos here.

1. https://github.com/knative/serving/pull/15414
2. https://github.com/knative/serving/pull/14454

Additional info