Mention behaviour of renew button being disabled

doc/subscriptions/gitlab_com/index.md

@@ -219,6 +219,8 @@ of the date of expiry with a banner in the GitLab user interface.
 To renew your subscription:
 
 1. Log in to the [Customers Portal](https://customers.gitlab.com/customers/sign_in) and beneath your existing subscription, select **Renew**.
+The **Renew** button remains disabled (grayed-out) until 15 days before a subscription expires.
+You can hover your mouse on the **Renew** button to see the date when it will become active.
 1. Review your renewal details and complete the payment process.
 1. Select **Confirm purchase**.
 

doc/subscriptions/self_managed/index.md

@@ -384,6 +384,8 @@ We recommend following these steps during renewal:
 1. Prune any inactive or unwanted users by [blocking them](../../user/admin_area/moderate_users.md#block-a-user).
 1. Determine if you have a need for user growth in the upcoming subscription.
 1. Log in to the [Customers Portal](https://customers.gitlab.com/customers/sign_in) and select the **Renew** button beneath your existing subscription.
+The **Renew** button remains disabled (grayed-out) until 15 days before a subscription expires.
+You can hover your mouse on the **Renew** button to see the date when it will become active.
 
    NOTE:
    If you need to change your [GitLab tier](https://about.gitlab.com/pricing/), contact our sales team via [the sales contact form](https://about.gitlab.com/sales/) for assistance as this can't be done in the Customers Portal.