feat(upnp): map ports through double NAT

docs/configuration.md

@@ -1553,6 +1553,13 @@ consent prompt for directory ACLs, so the Web UI shows setup steps for correctin
         <td>Description</td>
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             Sunshine will attempt to open ports for streaming over the internet.
+            If Sunshine's router is behind another router (double NAT) and both support UPnP,
+            the ports are also opened on the outer router, forwarded to the inner router.
+            Existing rules for another destination are left unchanged, and only rules owned by Sunshine are removed.
+            A disabled rule created by someone else is also left unchanged, and its port is reported as not opened.
+            Rules without an expiry are used only when the router explicitly requires them; they are recorded in
+            <code>upnp_mappings.journal</code> in Sunshine's application data directory before creation.
+            At the next start Sunshine attempts to remove outstanding recorded rules, even if UPnP has since been disabled.
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