add doc for spark.streaming.stopGracefullyOnShutdown

[wb14123]

No description provided.

[tdas]

from shutdown hook --> from JVM shutdown hook
streamingContext -> StreamingContext
streamingContext shutdown gracefully --> streamingContext stop gracefully from shutdown hook.

[tdas]

ok to test

[SparkQA]

Test build #42957 has finished for PR 8898 at commit cdc7448.

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

[SparkQA]

Test build #42961 has finished for PR 8898 at commit a2e7f75.

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

[srowen]

The text reads funny here. Maybe just: "If true, Spark shuts down the StreamingContext gracefully on JVM shutdown rather than immediately."

[wb14123]

It sounds better. I'm not a native English speaker so I'll modify it as your suggestion.

[SparkQA]

Test build #42968 has finished for PR 8898 at commit 98c9f57.

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

[srowen]

LGTM