[SPARK-18188][DOC][FOLLOW-UP]Add spark.broadcast.checksum to configuration

[10110346]

What changes were proposed in this pull request?

This pr add spark.broadcast.checksum to configuration.

How was this patch tested?

manually tested

[SparkQA]

Test build #93329 has finished for PR 21825 at commit 6a85aad.

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

[srowen]

Are you sure this is supposed to be documented for end users?

[10110346]

I think it's necessary to let the end users know:
1.this feature is already stable
2.People can disable it if their network is stable, doing this is good for performance
@srowen thanks

[srowen]

If this is going to be documented, it should be documented in a way that's useful for end users. This typically means describing what turning it on and off does from an end-user perspective. Does it make things faster, slower? more reliable, compatible, etc? This description here pretty much reiterates what's in the property key itself.

[SparkQA]

Test build #93823 has finished for PR 21825 at commit cf27272.

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

[srowen]

Nits like: space after a period, and the default is already documented above. I think this could still be more useful. What about: "If enabled, broadcasts will include a checksum, which can help detect corrupted blocks, at the cost of computing and sending a little more data. It's possible to disable it if the network has other mechanisms to guarantee data won't be corrupted during broadcast."

CC @davies . I guess even I'm not sure when I would disable this ... what would a network have to guarantee to avoid whatever corruption is possible here? Here it isn't clear yet when it's safe, when it won't lead to correctness issues.

[SparkQA]

Test build #93876 has finished for PR 21825 at commit d18d1b2.

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

[srowen]

Final nit: add the space after the period.

[SparkQA]

Test build #93919 has finished for PR 21825 at commit 09cbe26.

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

[srowen]

Merged to master

[10110346]

Thanks @srowen