Revert my previous change to restore the original collision group example

Reverting my change (Roblox#984) because the original example is clearer and handles all cases better.
It properly sets collision groups for parts added after the character spawns, which my version didn’t cover.
This makes the official documentation more complete and easier to understand.

Regarding performance, the difference is minimal for most use cases, but the original version is safer as it handles dynamic parts correctly without impacting performance noticeably.

Author: Lucatacos

content/en-us/workspace/collisions.md

@@ -270,27 +270,27 @@ interesting but unintended gameplay, such as characters jumping on top of each o
 local PhysicsService = game:GetService("PhysicsService")
 local Players = game:GetService("Players")
 
-local CollisionGroupName = "Characters"
-PhysicsService:RegisterCollisionGroup(CollisionGroupName)
-PhysicsService:CollisionGroupSetCollidable(CollisionGroupName, CollisionGroupName, false)
-
-local function setCollisionGroup(model)
-  -- Apply collision group to all existing parts in the model
-  for _, descendant in model:GetDescendants() do
-    if descendant:IsA("BasePart") then
-      descendant.CollisionGroup = CollisionGroupName
-    end
-  end
+PhysicsService:RegisterCollisionGroup("Characters")
+PhysicsService:CollisionGroupSetCollidable("Characters", "Characters", false)
+
+local function onDescendantAdded(descendant)
+	-- Set collision group for any part descendant
+	if descendant:IsA("BasePart") then
+		descendant.CollisionGroup = "Characters"
+	end
+end
+
+local function onCharacterAdded(character)
+	-- Process existing and new descendants for physics setup
+	for _, descendant in character:GetDescendants() do
+		onDescendantAdded(descendant)
+	end
+	character.DescendantAdded:Connect(onDescendantAdded)
 end
 
 Players.PlayerAdded:Connect(function(player)
-  player.CharacterAdded:Connect(function(character)
-    setCollisionGroup(character)
-  end)
-  -- If the player already has a character, apply the collision group immediately
-  if player.Character then
-    setCollisionGroup(player.Character)
-  end
+	-- Detect when the player's character is added
+	player.CharacterAdded:Connect(onCharacterAdded)
 end)
 ```