Add description for unowned parts in network ownership (#1384)

## Changes

<!-- Please summarize your changes. -->
The network ownership article does not document what black debug
visualization means, I have added documentation for that here.

<!-- Please link to any applicable information (forum posts, bug
reports, etc.). -->
https://devforum.roblox.com/t/what-is-black-network-ownership/2872112

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Author: Ticko-Grey

content/en-us/physics/network-ownership.md

@@ -89,6 +89,11 @@ To assist with network ownership debugging, Studio can render colored outlines a
 		<td>(white/grey)</td>
 		<td>Server or another client owns the part through either automatic network ownership or from explicit assignment through `Class.BasePart:SetNetworkOwner()|part:SetNetworkOwner()`.</td>
 	</tr>
+	<tr>
+		<td><ColorSwatch value="rgb(0,0,0)" /></td>
+		<td>(black)</td>
+		<td>The part is owned by neither the server nor the client. Physics will not apply to this part.</td>
+	</tr>
 </tbody>
 </table>