[CI] Update crossbow message about private org visibility

[assignUser]

The default membership in the ASF org is private which is a bit hard to spot as there is no indication of it being hidden and you can see other members badge as well. We should a link to the GH docs to the reflection message.

Originally posted by @lidavidm in #39215 (comment)

[github-actions]

This issue has been marked as stale because it has had no activity in the past 365 days. Please remove the stale label or comment below, or this issue will be closed in 14 days. If this improvement is still desired but has no current owner, please add the 'Status: needs champion' label.

[thisisnic]

This issue has been closed because there has been no activity for the 14 days since the stale warning. Feel free to reopen if this has been closed in error.