### What?
This PR adds back clarification that the `secret` configured in Payload
is not used directly to sign JWT tokens, which can cause confusion when
attempting to verify tokens in other services.
### Why?
There was previously an issue (#2441) that explained this unexpected
behavior, and documentation was added to clarify it. However, that
clarification has since been removed from the current docs, which led to
confusion when I attempted to validate the JWT in another service and
received an "invalid signature" error.
### How?
- Included a brief explanation and a cautionary warning to inform
developers of this custom behavior.
Fixes#13814
0a5b7b0485