DOC: Update merge copy parameter codes and doc

Signed-off-by: Liang Yan <[email protected]>

pandas/core/frame.py

@@ -361,7 +361,8 @@
     `right` should be left as-is, with no suffix. At least one of the
     values must not be None.
 copy : bool, default True
-    If False, avoid copy if possible.
+    If False, avoid copy if possible. If CoW is used, then the copy will be set
+    to False. If merge finally work on axis 0, then copy operation may be needed.
 indicator : bool or str, default False
     If True, adds a column to the output DataFrame called "_merge" with
     information on the source of each row. The column can be given a different

pandas/core/internals/concat.py

@@ -186,7 +186,7 @@ def concatenate_managers(
 
             fastpath = blk.values.dtype == values.dtype
         else:
-            values = _concatenate_join_units(join_units, copy=copy)
+            values = _concatenate_join_units(join_units)
             fastpath = False
 
         if fastpath:
@@ -454,7 +454,7 @@ def get_reindexed_values(self, empty_dtype: DtypeObj, upcasted_na) -> ArrayLike:
             return self.block.values
 
 
-def _concatenate_join_units(join_units: list[JoinUnit], copy: bool) -> ArrayLike:
+def _concatenate_join_units(join_units: list[JoinUnit]) -> ArrayLike:
     """
     Concatenate values from several join units along axis=1.
     """

pandas/core/reshape/concat.py

@@ -628,7 +628,6 @@ def get_result(self):
             # stack blocks
             if self.bm_axis == 0:
                 name = com.consensus_name_attr(self.objs)
-                cons = sample._constructor
 
                 arrs = [ser._values for ser in self.objs]