[SPARK-30091][SQL][PYTHON] Document mergeSchema option directly in the PySpark Parquet APIs

### What changes were proposed in this pull request?

This change properly documents the `mergeSchema` option directly in the Python APIs for reading Parquet data.

### Why are the changes needed?

The docstring for `DataFrameReader.parquet()` mentions `mergeSchema` but doesn't show it in the API. It seems like a simple oversight.

Before this PR, you'd have to do this to use `mergeSchema`:

```python
spark.read.option('mergeSchema', True).parquet('test-parquet').show()
```

After this PR, you can use the option as (I believe) it was intended to be used:

```python
spark.read.parquet('test-parquet', mergeSchema=True).show()
```

### Does this PR introduce any user-facing change?

Yes, this PR changes the signatures of `DataFrameReader.parquet()` and `DataStreamReader.parquet()` to match their docstrings.

### How was this patch tested?

Testing the `mergeSchema` option directly seems to be left to the Scala side of the codebase. I tested my change manually to confirm the API works.

I also confirmed that setting `spark.sql.parquet.mergeSchema` at the session does not get overridden by leaving `mergeSchema` at its default when calling `parquet()`:

```
>>> spark.conf.set('spark.sql.parquet.mergeSchema', True)
>>> spark.range(3).write.parquet('test-parquet/id')
>>> spark.range(3).withColumnRenamed('id', 'name').write.parquet('test-parquet/name')
>>> spark.read.option('recursiveFileLookup', True).parquet('test-parquet').show()
+----+----+
|  id|name|
+----+----+
|null|   1|
|null|   2|
|null|   0|
|   1|null|
|   2|null|
|   0|null|
+----+----+
>>> spark.read.option('recursiveFileLookup', True).parquet('test-parquet', mergeSchema=False).show()
+----+
|  id|
+----+
|null|
|null|
|null|
|   1|
|   2|
|   0|
+----+
```

Closes apache#26730 from nchammas/parquet-merge-schema.

Authored-by: Nicholas Chammas <[email protected]>
Signed-off-by: HyukjinKwon <[email protected]>

Author: nchammas

python/pyspark/sql/readwriter.py

@@ -305,22 +305,22 @@ def table(self, tableName):
 
     @since(1.4)
     def parquet(self, *paths, **options):
-        """Loads Parquet files, returning the result as a :class:`DataFrame`.
+        """
+        Loads Parquet files, returning the result as a :class:`DataFrame`.
 
+        :param mergeSchema: sets whether we should merge schemas collected from all
+            Parquet part-files. This will override ``spark.sql.parquet.mergeSchema``.
+            The default value is specified in ``spark.sql.parquet.mergeSchema``.
         :param recursiveFileLookup: recursively scan a directory for files. Using this option
                                     disables `partition discovery`_.
 
-        You can set the following Parquet-specific option(s) for reading Parquet files:
-            * ``mergeSchema``: sets whether we should merge schemas collected from all \
-                Parquet part-files. This will override ``spark.sql.parquet.mergeSchema``. \
-                The default value is specified in ``spark.sql.parquet.mergeSchema``.
-
         >>> df = spark.read.parquet('python/test_support/sql/parquet_partitioned')
         >>> df.dtypes
         [('name', 'string'), ('year', 'int'), ('month', 'int'), ('day', 'int')]
         """
+        mergeSchema = options.get('mergeSchema', None)
         recursiveFileLookup = options.get('recursiveFileLookup', None)
-        self._set_opts(recursiveFileLookup=recursiveFileLookup)
+        self._set_opts(mergeSchema=mergeSchema, recursiveFileLookup=recursiveFileLookup)
         return self._df(self._jreader.parquet(_to_seq(self._spark._sc, paths)))
 
     @ignore_unicode_prefix

python/pyspark/sql/streaming.py

@@ -535,26 +535,25 @@ def orc(self, path, recursiveFileLookup=None):
             raise TypeError("path can be only a single string")
 
     @since(2.0)
-    def parquet(self, path, recursiveFileLookup=None):
-        """Loads a Parquet file stream, returning the result as a :class:`DataFrame`.
+    def parquet(self, path, mergeSchema=None, recursiveFileLookup=None):
+        """
+        Loads a Parquet file stream, returning the result as a :class:`DataFrame`.
+
+        .. note:: Evolving.
 
+        :param mergeSchema: sets whether we should merge schemas collected from all
+            Parquet part-files. This will override ``spark.sql.parquet.mergeSchema``.
+            The default value is specified in ``spark.sql.parquet.mergeSchema``.
         :param recursiveFileLookup: recursively scan a directory for files. Using this option
                                     disables `partition discovery`_.
 
-        You can set the following Parquet-specific option(s) for reading Parquet files:
-            * ``mergeSchema``: sets whether we should merge schemas collected from all \
-                Parquet part-files. This will override ``spark.sql.parquet.mergeSchema``. \
-                The default value is specified in ``spark.sql.parquet.mergeSchema``.
-
-        .. note:: Evolving.
-
         >>> parquet_sdf = spark.readStream.schema(sdf_schema).parquet(tempfile.mkdtemp())
         >>> parquet_sdf.isStreaming
         True
         >>> parquet_sdf.schema == sdf_schema
         True
         """
-        self._set_opts(recursiveFileLookup=recursiveFileLookup)
+        self._set_opts(mergeSchema=mergeSchema, recursiveFileLookup=recursiveFileLookup)
         if isinstance(path, basestring):
             return self._df(self._jreader.parquet(path))
         else: