[SPARK-49716][PS][DOCS][TESTS] Fix documentation and add test of barh…

… plot

### What changes were proposed in this pull request?
- Update the documentation for barh plot to clarify the difference between axis interpretation in Plotly and Matplotlib.
- Test multiple columns as value axis.

The parameter difference is demonstrated as below.
```py
>>> df = ps.DataFrame({'lab': ['A', 'B', 'C'], 'val': [10, 30, 20]})
>>> df.plot.barh(x='val', y='lab').show()  # plot1

>>> ps.set_option('plotting.backend', 'matplotlib')
>>> import matplotlib.pyplot as plt
>>> df.plot.barh(x='lab', y='val')
>>> plt.show()  # plot2
```

plot1
![newplot (5)](https://github.com/user-attachments/assets/f1b6fabe-9509-41bb-8cfb-0733f65f1643)

plot2
![Figure_1](https://github.com/user-attachments/assets/10e1b65f-6116-4490-9956-29e1fbf0c053)

### Why are the changes needed?
The barh plot’s x and y axis behavior differs between Plotly and Matplotlib, which may confuse users. The updated documentation and tests help ensure clarity and prevent misinterpretation.

### Does this PR introduce _any_ user-facing change?
No. Doc change only.

### How was this patch tested?
Unit tests.

### Was this patch authored or co-authored using generative AI tooling?
No.

Closes apache#48161 from xinrong-meng/ps_barh.

Authored-by: Xinrong Meng <xinrong@apache.org>
Signed-off-by: Dongjoon Hyun <dongjoon@apache.org>

python/pyspark/pandas/plot/core.py

@@ -756,10 +756,10 @@ def barh(self, x=None, y=None, **kwargs):
 
         Parameters
         ----------
-        x : label or position, default DataFrame.index
-            Column to be used for categories.
-        y : label or position, default All numeric columns in dataframe
+        x : label or position, default All numeric columns in dataframe
             Columns to be plotted from the DataFrame.
+        y : label or position, default DataFrame.index
+            Column to be used for categories.
         **kwds
             Keyword arguments to pass on to
             :meth:`pyspark.pandas.DataFrame.plot` or :meth:`pyspark.pandas.Series.plot`.
@@ -770,6 +770,13 @@ def barh(self, x=None, y=None, **kwargs):
             Return an custom object when ``backend!=plotly``.
             Return an ndarray when ``subplots=True`` (matplotlib-only).
 
+        Notes
+        -----
+        In Plotly and Matplotlib, the interpretation of `x` and `y` for `barh` plots differs.
+        In Plotly, `x` refers to the values and `y` refers to the categories.
+        In Matplotlib, `x` refers to the categories and `y` refers to the values.
+        Ensure correct axis labeling based on the backend used.
+
         See Also
         --------
         plotly.express.bar : Plot a vertical bar plot using plotly.

python/pyspark/pandas/tests/plot/test_frame_plot_plotly.py

@@ -105,9 +105,10 @@ def check_barh_plot_with_x_y(pdf, psdf, x, y):
             self.assertEqual(pdf.plot.barh(x=x, y=y), psdf.plot.barh(x=x, y=y))
 
         # this is testing plot with specified x and y
-        pdf1 = pd.DataFrame({"lab": ["A", "B", "C"], "val": [10, 30, 20]})
+        pdf1 = pd.DataFrame({"lab": ["A", "B", "C"], "val": [10, 30, 20], "val2": [1.1, 2.2, 3.3]})
         psdf1 = ps.from_pandas(pdf1)
-        check_barh_plot_with_x_y(pdf1, psdf1, x="lab", y="val")
+        check_barh_plot_with_x_y(pdf1, psdf1, x="val", y="lab")
+        check_barh_plot_with_x_y(pdf1, psdf1, x=["val", "val2"], y="lab")
 
     def test_barh_plot(self):
         def check_barh_plot(pdf, psdf):