[SPARK-37335][ML] Flesh out FPGrowth docs

[nchammas]

What changes were proposed in this pull request?

Some of the columns included in the DataFrames returned by FPGrowthModel are not explained. This PR adds some basic documentation for them. Since those docs depend on the concept of an itemset, this PR also introduces that term.

Why are the changes needed?

The FPGrowth docs should cover at a high level all the basic information that the algorithm provides.

Does this PR introduce any user-facing change?

Yes, it updates user-facing documentation.

How was this patch tested?

I will confirm the doc changes are correct as part of PR review.

[nchammas]

cc @srowen since you also reviewed #22236.

(I don't mean to imply "you touch it, you own it"; I'm just not sure who else would know/care about FPGrowth. 😄)

[SparkQA]

Test build #145245 has finished for PR 34605 at commit e349eb4.

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

[SparkQA]

Kubernetes integration test starting
URL: https://amplab.cs.berkeley.edu/jenkins/job/SparkPullRequestBuilder-K8s/49715/

[SparkQA]

Kubernetes integration test status failure
URL: https://amplab.cs.berkeley.edu/jenkins/job/SparkPullRequestBuilder-K8s/49715/

[srowen]

Merged to master