[SPARK-22484][DOC] Document PySpark DataFrame csv writer behavior whe…

## What changes were proposed in this pull request?

In PySpark API Document, DataFrame.write.csv() says that setting the quote parameter to an empty string should turn off quoting. Instead, it uses the [null character](https://en.wikipedia.org/wiki/Null_character) as the quote.

This PR fixes the doc.

## How was this patch tested?

Manual.

```
cd python/docs
make html
open _build/html/pyspark.sql.html
```

Author: gaborgsomogyi <[email protected]>

Closes #19814 from gaborgsomogyi/SPARK-22484.

Author: gaborgsomogyi

python/pyspark/sql/readwriter.py

@@ -828,8 +828,7 @@ def csv(self, path, mode=None, compression=None, sep=None, quote=None, escape=No
                     set, it uses the default value, ``,``.
         :param quote: sets the single character used for escaping quoted values where the
                       separator can be part of the value. If None is set, it uses the default
-                      value, ``"``. If you would like to turn off quotations, you need to set an
-                      empty string.
+                      value, ``"``. If an empty string is set, it uses ``u0000`` (null character).
         :param escape: sets the single character used for escaping quotes inside an already
                        quoted value. If None is set, it uses the default value, ``\``
         :param escapeQuotes: a flag indicating whether values containing quotes should always

sql/core/src/main/scala/org/apache/spark/sql/DataFrameWriter.scala

@@ -592,7 +592,8 @@ final class DataFrameWriter[T] private[sql](ds: Dataset[T]) {
    * <li>`sep` (default `,`): sets the single character as a separator for each
    * field and value.</li>
    * <li>`quote` (default `"`): sets the single character used for escaping quoted values where
-   * the separator can be part of the value.</li>
+   * the separator can be part of the value. If an empty string is set, it uses `u0000`
+   * (null character).</li>
    * <li>`escape` (default `\`): sets the single character used for escaping quotes inside
    * an already quoted value.</li>
    * <li>`escapeQuotes` (default `true`): a flag indicating whether values containing