[SPARK-10676] [docs] Add documentation for SASL encryption options.

[vanzin]

No description provided.

[JoshRosen]

Should we also explicitly say that encryption is not yet supported for shuffle data at rest (e.g. in spill files, shuffle files, data cached in memory or on-disk, etc)?

[vanzin]

Ok, I'll add something.

[SparkQA]

Test build #42623 has finished for PR 8803 at commit d170730.

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

[SparkQA]

Test build #42626 has finished for PR 8803 at commit f2e9515.

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

[SparkQA]

Test build #42634 has finished for PR 8803 at commit 8db3b07.

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

[JoshRosen]

These changes look good to me, but I have one more minor ask: can you copy the docs for these configs to the "Security" section of the master configuration documentation page? https://spark.apache.org/docs/latest/configuration.html#security

[SparkQA]

Test build #42700 has finished for PR 8803 at commit f656dfa.

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

[JoshRosen]

LGTM, so I'm going to merge this into master and branch-1.5. Thanks Marcelo!