[neuralynx] add missing docstring parameter

Author: sprenger

neo/io/neuralynxio.py

@@ -35,6 +35,9 @@ def __init__(self, dirname='', filename='', use_cache=False, cache_path='same_as
         ----------
         dirname : str
             Directory containing data files
+        filename : str
+            Name of a single ncs, nse, nev, or ntt file to include in dataset. Will be ignored,
+            if dirname is provided.
         use_cache : bool, optional
             Cache results of initial file scans for faster loading in subsequent runs.
             Default: False