[MINOR][DOC] Use raw triple double quotes around docstrings where there are occurrences of backslashes.

From [PEP 257](https://www.python.org/dev/peps/pep-0257/):

> For consistency, always use """triple double quotes""" around docstrings. Use r"""raw triple double quotes""" if you use any backslashes in your docstrings. For Unicode docstrings, use u"""Unicode triple-quoted strings""".

For example, this is what help (kafka_wordcount) shows:

```
DESCRIPTION
    Counts words in UTF8 encoded, '
    ' delimited text received from the network every second.
     Usage: kafka_wordcount.py <zk> <topic>

     To run this on your local machine, you need to setup Kafka and create a producer first, see
     http://kafka.apache.org/documentation.html#quickstart

     and then run the example
        `$ bin/spark-submit --jars       external/kafka-assembly/target/scala-*/spark-streaming-kafka-assembly-*.jar       examples/src/main/python/streaming/kafka_wordcount.py       localhost:2181 test`
```

This is what it shows, after the fix:

```
DESCRIPTION
    Counts words in UTF8 encoded, '\n' delimited text received from the network every second.
    Usage: kafka_wordcount.py <zk> <topic>

    To run this on your local machine, you need to setup Kafka and create a producer first, see
    http://kafka.apache.org/documentation.html#quickstart

    and then run the example
       `$ bin/spark-submit --jars \
         external/kafka-assembly/target/scala-*/spark-streaming-kafka-assembly-*.jar \
         examples/src/main/python/streaming/kafka_wordcount.py \
         localhost:2181 test`
```

The thing worth noticing is no linebreak here in the help.

## What changes were proposed in this pull request?

Change triple double quotes to raw triple double quotes when there are occurrences of backslashes in docstrings.

## How was this patch tested?

Manually as this is a doc fix.

Author: Shashwat Anand <me@shashwat.me>

Closes #20497 from ashashwat/docstring-fixes.

examples/src/main/python/sql/streaming/structured_network_wordcount.py

@@ -15,7 +15,7 @@
 # limitations under the License.
 #
 
-"""
+r"""
  Counts words in UTF8 encoded, '\n' delimited text received from the network.
  Usage: structured_network_wordcount.py <hostname> <port>
    <hostname> and <port> describe the TCP server that Structured Streaming

examples/src/main/python/sql/streaming/structured_network_wordcount_windowed.py

@@ -15,7 +15,7 @@
 # limitations under the License.
 #
 
-"""
+r"""
  Counts words in UTF8 encoded, '\n' delimited text received from the network over a
  sliding window of configurable duration. Each line from the network is tagged
  with a timestamp that is used to determine the windows into which it falls.

examples/src/main/python/streaming/direct_kafka_wordcount.py

@@ -15,7 +15,7 @@
 # limitations under the License.
 #
 
-"""
+r"""
  Counts words in UTF8 encoded, '\n' delimited text directly received from Kafka in every 2 seconds.
  Usage: direct_kafka_wordcount.py <broker_list> <topic>
 

examples/src/main/python/streaming/flume_wordcount.py

@@ -15,7 +15,7 @@
 # limitations under the License.
 #
 
-"""
+r"""
  Counts words in UTF8 encoded, '\n' delimited text received from the network every second.
  Usage: flume_wordcount.py <hostname> <port>
 

examples/src/main/python/streaming/kafka_wordcount.py

@@ -15,7 +15,7 @@
 # limitations under the License.
 #
 
-"""
+r"""
  Counts words in UTF8 encoded, '\n' delimited text received from the network every second.
  Usage: kafka_wordcount.py <zk> <topic>
 

examples/src/main/python/streaming/network_wordcount.py

@@ -15,7 +15,7 @@
 # limitations under the License.
 #
 
-"""
+r"""
  Counts words in UTF8 encoded, '\n' delimited text received from the network every second.
  Usage: network_wordcount.py <hostname> <port>
    <hostname> and <port> describe the TCP server that Spark Streaming would connect to receive data.

examples/src/main/python/streaming/network_wordjoinsentiments.py

@@ -15,7 +15,7 @@
 # limitations under the License.
 #
 
-"""
+r"""
  Shows the most positive words in UTF8 encoded, '\n' delimited text directly received the network
  every 5 seconds. The streaming data is joined with a static RDD of the AFINN word list
  (http://neuro.imm.dtu.dk/wiki/AFINN)

examples/src/main/python/streaming/sql_network_wordcount.py

@@ -15,7 +15,7 @@
 # limitations under the License.
 #
 
-"""
+r"""
  Use DataFrames and SQL to count words in UTF8 encoded, '\n' delimited text received from the
  network every second.
 

examples/src/main/python/streaming/stateful_network_wordcount.py

@@ -15,7 +15,7 @@
 # limitations under the License.
 #
 
-"""
+r"""
  Counts words in UTF8 encoded, '\n' delimited text received from the
  network every second.