Update flag info for custom streaming headers

Max Woolfrequested to merge
mw/docs/custom-streaming-headers into master

What does this MR do and why?

Updates flag documentation for streaming_audit_event_headers (#362941 (closed)) now that the feature is available on .com

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

Merge request reports