[Documentation] Describe the difference between expression-locals and object-locals

[sbarzowski]

There were questions about it on Slack and Stack Overflow recently.

https://stackoverflow.com/questions/51211468/how-to-use-local-variables-in-arrays
https://kubernetes.slack.com/archives/C6JLE4L9X/p1529178734000072

[sbarzowski]

Reference section seems like the most appropriate place for that.