extended batch "dir:" description

docs/batch.md

@@ -55,6 +55,7 @@ A complete grammar is defined further below. The main components of a batch file
 dir
 : The base directory for all contained paths. Consider the example above. The path `instance1.cnf` will be expanded to `./some/path/instance1.cnf`. 
 : If the base path of a batch is relative (e.g. `./path`) it is always relative to its parent batch (e.g. `./nested/path` will be extended to `./some/path/nested/path`). In case of root batches (highest level in batch file) relative base paths are relative to the file location.
+: If no `"dir:"` is specified, the base path of the parent batch is inherited (no `"dir:"` is similar to `"dir": "."`).
 
 include
 : Here you can list paths to other batch files, which than will be included to the current batch.