feat: configurable notification level: add threshold example doc

Author: alex-courtis

doc/nvim-tree-lua.txt

@@ -1048,6 +1048,11 @@ Configuration for notification.
     Specify minimum notification level, uses the values from |vim.log.levels|
       Type: `enum`, Default: `vim.log.levels.INFO`
 
+    `ERROR`:   hard errors e.g. failure to read from the file system.
+    `WARNING`: non-fatal errors e.g. unable to system open a file.
+    `INFO:`    information only e.g. file copy path confirmation.
+    `DEBUG:`   not used.
+
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  4.1 VINEGAR STYLE                                         *nvim-tree-vinegar*