[SaaS] Call out SNI host header not available with O2O (#20869)

* [SaaS] Update custom-origin.mdx

* Apply suggestion from code review

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Co-authored-by: Rebecca Tamachiro <[email protected]>

Author: ngayerie

src/content/docs/cloudflare-for-platforms/cloudflare-for-saas/start/advanced-settings/custom-origin.mdx

@@ -54,6 +54,8 @@ Choose how your custom hostname populates the SNI value with SNI rewrites:
 
 * Currently, SNI Rewrite is not supported for wildcard custom hostnames. Subdomains covered by a wildcard custom hostname send the custom origin server name as the SNI value.
 
+* In the [O2O context](/cloudflare-for-platforms/cloudflare-for-saas/saas-customers/how-it-works/) (when requests are originating from a proxied hostname on a zone also on Cloudflare), changing the SNI value to use host header is currently not supported.
+
 * SNI overrides defined in an [Origin Rule](/rules/origin-rules/) will take precedence over SNI rewrites.
 
 * SNI Rewrite usage is subject to the [Service-Specific Terms](https://www.cloudflare.com/service-specific-terms-application-services/#ssl-for-saas-terms).