feat(help): note that server mods must not be mentioned

[spifory]

Summary

Seeing as people assume that because the Server Mod role is mentionable, then they are allowed to mention the role in help threads. This adds a small note stating that the author is not allowed to mention any member, including the server mods.

(The reason why I specifically stated server mods is because it is currently the only mentionable role in the nextcord guild)