docs: add comment for USE-JWT-COOKIE header

[robrap]

Although we may no longer need the USE-JWT-COOKIE
header, it could break ecommerce if this were
removed at this time. So, we are leaving
a comment so we'll see this in any searches, and
avoid updating for now.

Once all backends, including ecommerce, have edx-drf-extensions>=10.2.0, this could be removed.

See "[DEPR]: USE-JWT-COOKIE header" for more details:

• [DEPR]: USE-JWT-COOKIE header openedx/edx-drf-extensions#371

---

Make sure that the following steps are done before merging:

• Have a Site Reliability Engineer review the PR if you don't own all of the services impacted.
• If you are adding any new default values that need to be overridden when this change goes live, update internal repos and add an entry to the top of the CHANGELOG.
• Performed the appropriate testing.