Add targetRef Docs

[MorrisLaw]

Describe the feature request
Add docs for targetRef once the recent targetRef changes have been cherrypicked into 1.20.

Affected product area (please put an X in all that apply)

[ ] Configuration Infrastructure
[ X ] Docs
[ ] Installation
[ ] Networking
[ ] Performance and Scalability
[ ] Policies and Telemetry
[ ] Security
[ ] Test and Release
[ ] User Experience