Document pipelineid

[timja]

Summary

pipelineid allows you to override the branch name which is useful if you want to e.g. ring changes but not duplicate templates, an example of that is here:
https://github.com/JayDoubleu/aks-sds-deploy-copy/blob/3c268de32d8a7525735089e8919463fe39ff4fda/updatecli/weekly.d/aks-version.yaml#L4

https://github.com/JayDoubleu/aks-sds-deploy-copy/blob/3c268de32d8a7525735089e8919463fe39ff4fda/updatecli/weekly.d/aks-version.yaml#L39

updatecli/updatecli#156 (comment)

Basic example

https://github.com/JayDoubleu/aks-sds-deploy-copy/blob/3c268de32d8a7525735089e8919463fe39ff4fda/updatecli/weekly.d/aks-version.yaml#L5

Motivation

See updatecli/updatecli#156 (comment)