add more tests and modify comments

Author: charlesdong1991

pandas/core/reshape/merge.py

@@ -161,8 +161,9 @@ def merge_ordered(left, right, on=None,
         Interpolation method for data
     suffixes : 2-length sequence (tuple, list, ...)
         Suffix to apply to overlapping column names in the left and right
-        side, respectively. Except for tuple of (str, str), it also allows
-        tuple of (None, str) or (str, None)
+        side, respectively. And each element should be either a str or None,
+        and at least one of elements must not be str. Valid inputs are like:
+        (str, str), [str, str], (None, str), [str, None], [None, str] etc
     how : {'left', 'right', 'outer', 'inner'}, default 'outer'
         * left: use only keys from left frame (SQL: left outer join)
         * right: use only keys from right frame (SQL: right outer join)

pandas/tests/reshape/merge/test_merge.py

@@ -1452,7 +1452,9 @@ def test_merge_series(on, left_on, right_on, left_index, right_index, nm):
     (0.0, 0.0, ("_x", None), ["0.0_x", 0.0]),
     ("b", "b", (None, "_y"), ["b", "b_y"]),
     ("a", "a", ("_x", None), ["a_x", "a"]),
-    ("a", "b", ("_x", None), ["a", "b"])
+    ("a", "b", ("_x", None), ["a", "b"]),
+    ("a", "a", [None, "_x"], ["a", "a_x"]),
+    (0, 0, ["_a", None], ["0_a", 0])
 ])
 def test_merge_suffix(col1, col2, suffixes, expected_cols):
     # issue: 24782
@@ -1469,7 +1471,9 @@ def test_merge_suffix(col1, col2, suffixes, expected_cols):
 @pytest.mark.parametrize("suffixes", [
     (None, None),
     ('', None),
-    (None, '')
+    (None, ''),
+    [None, None],
+    ['', None]
 ])
 def test_merge_suffix_errors(suffixes):
     # issue: 24782