[SPARK-27687][SS] Rename Kafka consumer cache capacity conf and document caching

[gaborgsomogyi]

What changes were proposed in this pull request?

Kafka related Spark parameters has to start with spark.kafka. and not with spark.sql.. Because of this I've renamed spark.sql.kafkaConsumerCache.capacity.

Since Kafka consumer caching is not documented I've added this also.

How was this patch tested?

Existing + added unit test.

cd docs
SKIP_API=1 jekyll build

and manual webpage check.

[gaborgsomogyi]

cc @srowen

[SparkQA]

Test build #105348 has finished for PR 24590 at commit f43ee4e.

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

[HeartSaVioR]

LGTM

[SparkQA]

Test build #105375 has finished for PR 24590 at commit 38bc96d.

• This patch fails due to an unknown error code, -9.
• This patch merges cleanly.
• This patch adds no public classes.

[gaborgsomogyi]

retest this please

[SparkQA]

Test build #105377 has finished for PR 24590 at commit 38bc96d.

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

[dongjoon-hyun]

+1, LGTM. Thank you, @gaborgsomogyi and @srowen , @HeartSaVioR .
Merged to master.

[BdLearnerr]

@gaborgsomogyi@SparkQA , can anyone help me whats wrong here https://stackoverflow.com/questions/58456939/how-to-set-spark-consumer-cache-to-fix-kafkaconsumer-cache-hitting-max-capaci

[gaborgsomogyi]

@BdLearnerr I've answered but next time please use the mailing list.