Add deterministic sampling cost warning to browser SDK v7 upgrade guide (#37609)

* Add deterministic sampling cost warning to browser SDK v7 upgrade guide

* Address PR review: fix style issues in deterministic sampling warning

Author: BenoitZugmeyer

content/en/real_user_monitoring/guide/browser-sdk-upgrade.md

@@ -32,6 +32,8 @@ The system that tracks sessions has been rewritten to improve data reliability a
 
 Previously, the sampling decision was made once at session creation and persisted. In v7, it is computed on demand from the session ID and sample rate, making it consistent regardless of which page initializes the SDK. If you use different sampling rates across pages, those rates are applied consistently.
 
+<div class="alert alert-warning">Upgrading to v7 introduces deterministic sampling for distributed traces based on the RUM session ID. As a result, under RUM without Limits&trade;, the likelihood of indexing sessions that had sampled associated traces is significantly increased. More traces are retained by your existing Cross-Product Retention Filters, even without any configuration change.<br><br>If you have cross-product Retention Filters (for example, RUM-linked APM traces), you may see an <strong>increase in the volume of indexed spans</strong>, which could lead to <strong>higher costs</strong>. Review your Retention Filter configuration and estimated span volume after upgrading.</div>
+
 #### Session store key renamed
 
 The session storage key has changed from `_dd_s` to `_dd_s_v2` because the new session manager uses an incompatible storage format. On upgrade, existing sessions are automatically migrated from `_dd_s`.