man: add note about append-only mode to restic-unlock.1

Since restic unlock _removes_ locks, a user might wonder
whether this works in append-only mode, which prevents deletion of
data.  This change explicitly mentions in the man page that the deletion
of locks is an allowed exception.

cmd/restic/cmd_unlock.go

@@ -19,6 +19,8 @@ func newUnlockCommand(globalOptions *global.Options) *cobra.Command {
 		Long: `
 The "unlock" command removes stale locks that have been created by other restic processes.
 
+Removing locks works even with repositories served in append-only mode from restic's rest-server.
+
 EXIT STATUS
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