DOC: fix doc for crosstab with Categorical data input (#33783)

* fix doc for crosstab with Categorical data input

* fix doc for crosstab with Categorical data input

* put dropna=False in a separate ipython block

Author: GYHHAHA

doc/source/user_guide/reshaping.rst

@@ -471,16 +471,24 @@ If ``crosstab`` receives only two Series, it will provide a frequency table.
 
     pd.crosstab(df['A'], df['B'])
 
-Any input passed containing ``Categorical`` data will have **all** of its
-categories included in the cross-tabulation, even if the actual data does
-not contain any instances of a particular category.
+``crosstab`` can also be implemented
+to ``Categorical`` data.
 
 .. ipython:: python
 
     foo = pd.Categorical(['a', 'b'], categories=['a', 'b', 'c'])
     bar = pd.Categorical(['d', 'e'], categories=['d', 'e', 'f'])
     pd.crosstab(foo, bar)
 
+If you want to include **all** of data categories even if the actual data does
+not contain any instances of a particular category, you should set ``dropna=False``.
+
+For example:
+
+.. ipython:: python
+
+    pd.crosstab(foo, bar, dropna=False)
+
 Normalization
 ~~~~~~~~~~~~~