Add comment into jetpack system, to prevent future grid jetpack PRs (#30682)

Update SharedJetpackSystem.cs

Content.Shared/Movement/Systems/SharedJetpackSystem.cs

@@ -125,6 +125,8 @@ public abstract class SharedJetpackSystem : EntitySystem
 
     private bool CanEnableOnGrid(EntityUid? gridUid)
     {
+        // No and no again! Do not attempt to activate the jetpack on a grid with gravity disabled. You will not be the first or the last to try this.
+        // https://discord.com/channels/310555209753690112/310555209753690112/1270067921682694234
         return gridUid == null ||
                (!HasComp<GravityComponent>(gridUid));
     }