Document trial limit per payment method for Copilot (#59278)

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: sunbrye <56200261+sunbrye@users.noreply.github.com>

Author: Copilot

content/billing/concepts/product-billing/github-copilot-licenses.md

@@ -47,6 +47,7 @@ There are several ways to use {% data variables.product.prodname_copilot_short %
 * You must choose a monthly or yearly billing cycle and provide a payment method.
 * If you do not cancel before the end of the trial, it automatically converts to a paid plan.
 * You can cancel any time during the 30 days. If you cancel, you will not be charged and will keep access until the trial ends.
+* Free trials are limited to three per payment method. Additional trials will continue as paid subscriptions.
 
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