The documentation you are referencing is for credit memos, you'll see in that same doc the definition of a credit memo:
A credit memo is a document that shows the amount that is due the customer for a full or partial refund.
This is different than store credit which is only available in Magento Commerce edition, not open source
I see the line you referenced now. I suspect that has just found its way into the the Open Source documentation erroneously. Store credit definitely isn't a feature of Magento Open Source.