Update custom-cache-key.mdx

add link to full documentation on cache keys

Author: chris-martinelli

src/content/docs/cache/how-to/cache-rules/examples/custom-cache-key.mdx

@@ -27,3 +27,5 @@ import { Example, Render } from "~/components"
     * **Query string**: All query string parameters
 
 </Example>
+
+See [cache keys](/cache/how-to/cache-keys/) for more information on possible settings when configuring a custom cache key.