Clarifying that path keyword is mandatory for cache

What does this MR do and why?

Clarifying that path keyword is mandatory for cache to happen.

How to set up and validate locally

Create a job as follows:

myjob:
 cache:
   key: a1234
   policy: pull-push

You will notice that no caching happens

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.