Document AME-16764 - dynamic secret IDs

We need to document -AME-16764-. As per the google doc in the comment:

The secret ID for a multi-instance config is determined using the instance ID and a hardcoded suffix
The multi-instance components will look up the secrets using the instance name + suffix combo first, then fall back to a “default” + suffix secret ID for a second lookup
The global mappings would only offer the global singleton and multi-instance secret IDs
The realm mappings would only offer the realm level singleton and multi-instance secret IDs.

currently we only document the default/static

am.default.authentication.modules.persistentcookie.encryption
 am.default.authentication.modules.persistentcookie.signing

https://backstage.forgerock.com/docs/am/6.5/maintenance-guide/#secrets-persistent-cookie
https://backstage.forgerock.com/docs/am/6.5/authentication-guide/index.html#secrets-persistent-cookie 

but we need to mention the dynamic ones (per instance + suffix) too , for example: