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

[xinrong-meng]

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.

>>> 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

plot2

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.

[dongjoon-hyun]

+1, LGTM. Thank you, @xinrong-meng .
Merged to master.

[xinrong-meng]

Thank you @dongjoon-hyun !