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

### 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.

Closes #34605 from nchammas/SPARK-37335-fpgrowth-docs.

Authored-by: Nicholas Chammas <fbfd45890515f7eeeeabf87d7a80d3449c3bce83@gmail.com>
Signed-off-by: Sean Owen <d519fa33a62684a61ab04b24d26f83452d51b34c@gmail.com>

vinodkc/spark,vinodkc/spark,shaneknapp/spark,hvanhovell/spark,shaneknapp/spark,mahak/spark,witgo/spark,zero323/spark,shaneknapp/spark,nchammas/spark,zzcclp/spark,zzcclp/spark,rednaxelafx/apache-spark,wangyum/spark,rednaxelafx/apache-spark,shaneknapp/spark,vinodkc/spark,gengliangwang/spark,rednaxelafx/apache-spark,dongjoon-hyun/spark,WeichenXu123/spark,vinodkc/spark,ueshin/apache-spark,HyukjinKwon/spark,JoshRosen/spark,srowen/spark,witgo/spark,mahak/spark,nchammas/spark,shaneknapp/spark,xuanyuanking/spark,xuanyuanking/spark,srowen/spark,wangyum/spark,holdenk/spark,cloud-fan/spark,apache/spark,WeichenXu123/spark,apache/spark,cloud-fan/spark,rednaxelafx/apache-spark,hvanhovell/spark,holdenk/spark,mahak/spark,JoshRosen/spark,xuanyuanking/spark,holdenk/spark,gengliangwang/spark,srowen/spark,JoshRosen/spark,holdenk/spark,zero323/spark,srowen/spark,hvanhovell/spark,ueshin/apache-spark,wangyum/spark,HyukjinKwon/spark,JoshRosen/spark,vinodkc/spark,WeichenXu123/spark,cloud-fan/spark,JoshRosen/spark,nchammas/spark,zzcclp/spark,rednaxelafx/apache-spark,hvanhovell/spark,dongjoon-hyun/spark,ueshin/apache-spark,srowen/spark,shaneknapp/spark,shaneknapp/spark,ueshin/apache-spark,cloud-fan/spark,srowen/spark,holdenk/spark,cloud-fan/spark,nchammas/spark,holdenk/spark,WeichenXu123/spark,wangyum/spark,zzcclp/spark,gengliangwang/spark,apache/spark,HyukjinKwon/spark,gengliangwang/spark,dongjoon-hyun/spark,xuanyuanking/spark,zero323/spark,WeichenXu123/spark,zzcclp/spark,mahak/spark,nchammas/spark,dongjoon-hyun/spark,xuanyuanking/spark,witgo/spark,WeichenXu123/spark,HyukjinKwon/spark,ueshin/apache-spark,zzcclp/spark,zero323/spark,HyukjinKwon/spark,vinodkc/spark,gengliangwang/spark,apache/spark,zero323/spark,zero323/spark,gengliangwang/spark,apache/spark,zzcclp/spark,hvanhovell/spark,dongjoon-hyun/spark,zero323/spark,witgo/spark,wangyum/spark,witgo/spark,HyukjinKwon/spark,vinodkc/spark,hvanhovell/spark,nchammas/spark,mahak/spark,cloud-fan/spark,ueshin/apache-spark,mahak/spark,gengliangwang/spark,mahak/spark,witgo/spark,xuanyuanking/spark,dongjoon-hyun/spark,dongjoon-hyun/spark,HyukjinKwon/spark,rednaxelafx/apache-spark,srowen/spark,witgo/spark,apache/spark,wangyum/spark,JoshRosen/spark,WeichenXu123/spark,wangyum/spark,hvanhovell/spark,ueshin/apache-spark,JoshRosen/spark,nchammas/spark,apache/spark,cloud-fan/spark,xuanyuanking/spark,holdenk/spark,rednaxelafx/apache-spark