Add multi-recipient info as description to the field

Problem/Motivation

https://docs.drupalcommerce.org/commerce2/user-guide/orders/customer-emails says

Enter the store notification email address into the Send a copy of the receipt to this email: field. You can enter multiple comma separated addresses into this field eg "one@example.com, two@example.com, three@example.com"

That should be added as inline documentation description to the field.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet