[Security] Add credential note to load balancing documentation

[legrego]

In Kibana's Load Balancing documentation, we mention a number of settings which must be configured identically across instances.

We need to update this documentation to explain that each instance must do ONE of the following:

1. [Preferred] Provide credentials via elasticsearch.serviceAccountToken for each instance. Different tokens are permitted in this setup, but not required.
2. Provide credentials via elasticsearch.username and elasticsearch.password. When using this method, each instance MUST be configured with the same username/password combination.

APEX-92

[elasticmachine]

Pinging @elastic/kibana-security (Team:Security)