[docs] Clarify about pinging Github group in the documentation

[uilianries]

Motivation

After a few comments about the missing visibility of the pinging barbarians group, this PR tries to make it more explicit in case it looks non-existent due to the missing autocomplete.

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• Read the contributing guidelines
• Checked that this PR is not a duplicate: list of PRs by recipe
• Tested locally with at least one configuration using a recent version of Conan

[ericLemanissier]

Are you sure about this ? I could be mistaken, but as far as I remember, private teams cannot be pinged by people who are not members of said team.
I just tested it on #28538 (comment)

[jcar87]

the @conan-io/barbarians is not a secret team, however it does appear that it's only useable by team members: https://docs.github.com/en/organizations/organizing-members-into-teams/about-teams#team-visibility

Now I need to determine whether whenever I get notified, it's because it works - or because Im subscribed to most comment events...