DOC: Make document merge_cells=columns in to_excel

Author: ZKaoChi

pandas/core/generic.py

@@ -2211,8 +2211,10 @@ def to_excel(
             via the options ``io.excel.xlsx.writer`` or
             ``io.excel.xlsm.writer``.
 
-        merge_cells : bool, default True
-            Write MultiIndex and Hierarchical Rows as merged cells.
+        merge_cells : bool or 'columns', default False
+            Write MultiIndex and Hierarchical Rows as merged cells if True.
+            Merge MultiIndex column header cells only if 'columns'.
+
         {encoding_parameter}
         inf_rep : str, default 'inf'
             Representation for infinity (there is no native representation for