[SPARK-49532][DOCS][PS] Improve documentation of "plotting.sample_ratio" option

### What changes were proposed in this pull request?
The current documentation incorrectly suggests that "plotting.sample_ratio" **defaults** to "plotting.max_rows". In reality, if "plotting.sample_ratio" is not explicitly set, it is **derived** based on the ratio of "plotting.max_rows" to the dataset size.

### Why are the changes needed?
To avoid misunderstandings about how these options interact.

### Does this PR introduce _any_ user-facing change?
Doc change only.

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

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

Closes apache#48013 from xinrong-meng/doc_impr.

Authored-by: Xinrong Meng <[email protected]>
Signed-off-by: Hyukjin Kwon <[email protected]>

Author: xinrong-meng

python/pyspark/pandas/config.py

@@ -287,7 +287,8 @@ def validate(self, v: Any) -> None:
         doc=(
             "'plotting.sample_ratio' sets the proportion of data that will be plotted for sample-"
             "based plots such as `plot.line` and `plot.area`. "
-            "This option defaults to 'plotting.max_rows' option."
+            "If not set, it is derived from 'plotting.max_rows', by calculating the ratio of "
+            "'plotting.max_rows' to the total data size."
         ),
         default=None,
         types=(float, type(None)),