chore(docs): Update documentation for apply coupon endpoint

[xlisachan]

This pull request

• provides additional information to utilize the endpoint /oauth/subscriptions/coupon/apply

Issue that this pull request solves

Checklist

Put an x in the boxes that apply

• [x] My commit is GPG signed.
• [x] I have added necessary documentation (if appropriate).

[xlisachan]

Adding update to #17124