Updating ContainerSource, Sources list, and migration example

[nachocano]

Fixes #2332

Proposed Changes

• Updated ContainerSource example.
• Updated migration docs
• Updated Sources list... Removed hardcoded spec fields. Those are part of the API reference and are generated.

[nachocano]

/assign @vaikas for approval powers

[knative-prow-robot]

@nachocano: GitHub didn't allow me to assign the following users: for, approval, powers.

Note that only knative members, repo collaborators and people who have commented on this issue/PR can be assigned. Additionally, issues/PRs can only have 10 assignees at the same time.
For more information please see the contributor guide

Details

In response to this:

/assign @vaikas for approval powers

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.

[nachocano]

/assign @vaikas
for approval powers

[vaikas]

/lgtm
/approve

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: nachocano, vaikas

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Details

Needs approval from an approver in each of these files:

• docs/eventing/OWNERS [vaikas]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment