DOC: Add docstring to string_like method

Add docstring to string_like method

statsmodels/tools/validation/validation.py

@@ -91,9 +91,9 @@ def array_like(
     >>> a = array_like(pd.Series(x), 'x', ndim=1)
     >>> a.shape
     (4,)
+
     >>> type(a.orig)
     pandas.core.series.Series
-
     Squeezes singleton dimensions when required
     >>> x = np.array(x).reshape((4, 1))
     >>> a = array_like(x, 'x', ndim=1)
@@ -395,6 +395,26 @@ def float_like(value, name, optional=False, strict=False):
 
 
 def string_like(value, name, optional=False, options=None, lower=True):
+    """
+    Check if object is string-like and raise if not
+    Parameters
+    ----------
+    value : object
+        Value to verify
+    name : str
+        Variable name for exceptions
+    optional : bool
+        Flag indicating whether None is allowed
+    options : Sequence[str] or tuple[str]
+        Allowed values for input parameter `value`
+    lower : bool
+        Convert all case-based characters in `value` into lowercase
+
+    Returns
+    -------
+    validated : str
+        string_like
+    """
     if value is None:
         return None
     if not isinstance(value, str):