add availability note to sequence custom rules (cloudflare#25251)

Author: patriciasantaana

src/content/docs/api-shield/security/sequence-mitigation/custom-rules.mdx

@@ -22,6 +22,10 @@ Rules built using these custom rules are different from sequence mitigation rule
 
 ## Availability
 
+:::note
+<Render file="sequence-mitigation-availability" product="api-shield" />
+:::
+
 <Render file="sequence-rules-availability" product="bots" />
 
 ## Build a sequence custom rule