Add docstring and change sorting in join function for compatibility

Author: reidy-p

Diff for: pandas/_libs/lib.pyx

@@ -201,6 +201,20 @@ def item_from_zerodim(val: object) -> object:
 @cython.wraparound(False)
 @cython.boundscheck(False)
 def fast_unique_multiple(list arrays, sort: bool=True):
+    """
+    Generate a list of unique values from a list of arrays.
+
+    Parameters
+    ----------
+    list : array-like
+        A list of array-like objects
+    sort : boolean
+        Whether or not to sort the resulting unique list
+
+    Returns
+    -------
+    unique_list : list of unique values
+    """
     cdef:
         ndarray[object] buf
         Py_ssize_t k = len(arrays)

Diff for: pandas/core/indexes/base.py

@@ -3244,9 +3244,9 @@ def join(self, other, how='left', level=None, return_indexers=False,
         elif how == 'right':
             join_index = other
         elif how == 'inner':
-            join_index = self.intersection(other, sort=sort)
+            join_index = self.intersection(other, sort=False)
         elif how == 'outer':
-            join_index = self.union(other, sort=sort)
+            join_index = self.union(other)
 
         if sort:
             join_index = join_index.sort_values()