Refine documentation of the AppTemplateValuesSource

[gberche-orange]

What this PR does / why we need it:

Which issue(s) this PR fixes:

Missing CRD doc for template valuesFrom secret

See related slack thread https://kubernetes.slack.com/archives/CH8KCCKA5/p1673453880059389

Does this PR introduce a user-facing change?

CRD doc refines

See example secret in https://github.com/vmware-tanzu/carvel-kapp-controller/blob/ad24bdc3e41219b176f88280a469e9fd21979339/examples/simple-app-git/2.yml#L21-L31

Relates to https://github.com/vmware-tanzu/carvel/pull/613

Additional Notes for your reviewer:

Review Checklist:

• Follows the developer guidelines
• Relevant tests are added or updated
• Relevant docs in this repo added or updated
• Relevant carvel.dev docs added or updated in a separate PR and there's
  a link to that PR
• Code is at least as readable and maintainable as it was before this
  change

Additional documentation e.g., Proposal, usage docs, etc.:

[neil-hickey]

hey @gberche-orange . LGTM Thanks for the update! To get this PR merged, the crds need to be regenerated 👍

Please run './hack/build.sh', './hack/gen.sh', and './hack/gen-apiserver.sh' and 'git add' the generated file(s).

[gberche-orange]

thanks @neil-hickey , I've merged the doc update into #1061 as to avoid multiple crds updates and potential conflicts