doc: Release note for export watchonly wallet gui action

Author: achow101

doc/release-notes-gui-872.md

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+GUI
+---
+
+* A menu action has been added to allow creating a watchonly wallet file from
+  an existing descriptor wallet. This option mirrors the `exportwatchonlywallet`
+  RPC - the exported file can be imported to another node using the Restore
+  Wallet menu action.