[SPARK-32888][DOCS] Add user document about header flag and RDD as pa…

…th for reading CSV

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

This proposes to enhance user document of the API for loading a Dataset of strings storing CSV rows. If the header option is set to true, the API will remove all lines same with the header.

### Why are the changes needed?

This behavior can confuse users. We should explicitly document it.

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

No. Only doc change.

### How was this patch tested?

Only doc change.

Closes apache#29765 from viirya/SPARK-32888.

Authored-by: Liang-Chi Hsieh <viirya@gmail.com>
Signed-off-by: HyukjinKwon <gurwls223@apache.org>

python/pyspark/sql/readwriter.py

@@ -430,6 +430,9 @@ def csv(self, path, schema=None, sep=None, encoding=None, quote=None, escape=Non
                         character. By default (None), it is disabled.
         :param header: uses the first line as names of columns. If None is set, it uses the
                        default value, ``false``.
+                       .. note:: if the given path is a RDD of Strings, this header
+                       option will remove all lines same with the header if exists.
+
         :param inferSchema: infers the input schema automatically from data. It requires one extra
                        pass over the data. If None is set, it uses the default value, ``false``.
         :param enforceSchema: If it is set to ``true``, the specified or inferred schema will be

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

@@ -600,6 +600,9 @@ class DataFrameReader private[sql](sparkSession: SparkSession) extends Logging {
    * If the enforceSchema is set to `false`, only the CSV header in the first line is checked
    * to conform specified or inferred schema.
    *
+   * @note if `header` option is set to `true` when calling this API, all lines same with
+   * the header will be removed if exists.
+   *
    * @param csvDataset input Dataset with one CSV row per record
    * @since 2.2.0
    */