Clean up container documentation in spec.md (#2736)
Clean up container documentation in spec.md (#2736)

* Factor out container

* Address PR comments

Change-Id: I74fa930d29a72d872d3c2d7dcf8465345746446e

* Add documentation for container ports

Change-Id: I3025bd7bcb3c1b88bc9ab1fdb4c6878f91c3ca89

* containerPort is not optional

Change-Id: Ia73c37df4b72a1774b279160f684e80f6f320f14

* Document disallowed ports

Change-Id: I825dd63d7766c8af670999bb7697c3387e8b55d6

* Update ports comment to be more clear

Change-Id: Ia0fdb91f91c175b35c57f420a7efb515f86fd723

knative/serving,knative/serving