[MINOR][PYTHON][DOCS] Refine the docstring of VariantVal

### What changes were proposed in this pull request?
Refine the docstring of `VariantVal`

### Why are the changes needed?
avoid the wildcard import, which is not suggested

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

### How was this patch tested?
doctest

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

Closes #50002 from zhengruifeng/py_doc_variantval.

Authored-by: Ruifeng Zheng <ruifengz@apache.org>
Signed-off-by: Hyukjin Kwon <gurwls223@apache.org>

python/pyspark/sql/types.py

@@ -1730,11 +1730,13 @@ class VariantVal:
 
     Examples
     --------
-    >>> from pyspark.sql.functions import *
+    >>> from pyspark.sql import functions as sf
     >>> df = spark.createDataFrame([ {'json': '''{ "a" : 1 }'''} ])
-    >>> v = df.select(parse_json(df.json).alias("var")).collect()[0].var
+    >>> v = df.select(sf.parse_json(df.json).alias("var")).head().var
     >>> v.toPython()
     {'a': 1}
+    >>> v.toJson()
+    '{"a":1}'
     """
 
     def __init__(self, value: bytes, metadata: bytes):