[DOCS] Add disclaimer that X-Pack Security users inherit anonymous roles

[n0othing]

When Anonymous Access is enabled, all other users will inherit whichever roles you assign your anonymous user. We should document this behavior as it can cause some confusing results (e.g an explicit user is given access to indices X/Y/Z, but also has access to index A thanks to xpack.security.authc.anonymous.roles).

Ideally, _es_anonymous_user should be the least privileged user, but it's not always the case.

[elasticmachine]

Pinging @elastic/es-security

[jrodewig]

[docs issue triage]

Leaving open. This is still relevant:
https://www.elastic.co/guide/en/elastic-stack-overview/master/anonymous-access.html