[Spark-8343] [Streaming] [Docs] Improve Spark Streaming Guides.

[dusenberrymw]

This improves the Spark Streaming Guides by fixing broken links, rewording confusing sections, fixing typos, adding missing words, etc.

[JoshRosen]

Ah, looks like this just conflicted with #6781.

Just wanted to say that I'm a huge fan of these style of documentation cleanup patches: they're a lot of work but have a big impact since these docs are pretty widely consumed. 👍

[dusenberrymw]

@JoshRosen Thanks! Also, I fixed the conflicts for this request, so we should be good to go now.

[JoshRosen]

The wordsmithing seems fine to me, but I'll defer to @tdas to sign off on the technical changes / edits to code examples.

[dusenberrymw]

@JoshRosen Actually, I think the code example changes were brought in when I merged with #6781. I'm going to close this PR, and open another one that is merged better.

[SparkQA]

Test build #34832 has finished for PR 6800 at commit c246cb7.

• This patch fails Spark unit tests.
• This patch does not merge cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #34829 has finished for PR 6800 at commit 4b655ec.

• This patch passes all tests.
• This patch does not merge cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #34833 has finished for PR 6800 at commit 0ab72bc.

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