Clarify if/how enduser.role can have multiple roles

[swar8080]

Changes

Some role-based access control implementations allow assuming multiple roles. For example, in Java's Spring Security framework

Let me know if it makes sense to clarify that multiple roles are possible in the enduser.role attribute documentation

I noticed this when looking at a request to capture enduser.role with spring security auto-instrumentation (open-telemetry/opentelemetry-java-instrumentation#9400)

Merge requirement checklist

• CONTRIBUTING.md guidelines followed.
• CHANGELOG.md updated for non-trivial changes.
• schema-next.yaml updated with changes to existing conventions.

[linux-foundation-easycla]

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