[Streaming][Doc][Minor] Update the description of direct Kafka stream doc

[jerryshao]

With the merge of SPARK-8337, now the Python API has the same functionalities compared to Scala/Java, so here changing the description to make it more precise.

@zsxwing @tdas , please review, thanks a lot.

[SparkQA]

Test build #47490 has finished for PR 10246 at commit 5c5cd95.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[zsxwing]

LGTM

[zsxwing]

Thanks @jerryshao. Merging to master, 1.6, 1.5, 1.4.

[jerryshao]

Hi @zsxwing, one of the feature "message handler" is only supported in 1.6. So maybe better not merge back to the previous version :).

[zsxwing]

Hi @zsxwing, one of the feature "message handler" is only supported in 1.6. So maybe better not merge back to the previous version :).

Is message handler mentioned in the doc? I cannot find it.

[jerryshao]

No, it is not mentioned in the doc. What I mean is that for previous version, Python api still miss this piece of functionality, so correcting this description in the previous version is not exactly correct.

[zsxwing]

But it will be confusing if we say in Spark 1.4 for the Python API in the doc, but it doesn't in docs for 1.4 and 1.5.

We don't mention if we support a custom message handler in the doc. Even if Python api didn't support the message handler in 1.4 and 1.5, we can still say Python api supports the Kafka direct stream. Right?