Adding more detail on separator Args for CSVlogger

Currently the Args Separator  doesn't explain to take only the 1-char string as delimiter which may cause 'TypeError' if user uses more than 1-char string delimiter.

Author: RenuPatelGoogle

tf_keras/callbacks.py

@@ -3164,6 +3164,7 @@ class CSVLogger(Callback):
     Args:
         filename: Filename of the CSV file, e.g. `'run/log.csv'`.
         separator: String used to separate elements in the CSV file.
+                   Separator string ("delimiter") must be a 1-character string.
         append: Boolean. True: append if file exists (useful for continuing
             training). False: overwrite existing file.
     """