### What changes were proposed in this pull request?
This is a follow-up of https://github.com/apache/spark/pull/29160. We already removed the indeterministicity. This PR aims the following for the existing code base.
1. Add an explicit document to `DataFrameReader/DataFrameWriter`.
2. Add `toMap` to `CaseInsensitiveMap` in order to return `originalMap: Map[String, T]` because it's more consistent with the existing `case-sensitive key names` behavior for the existing code pattern like `AppendData.byName(..., extraOptions.toMap)`. Previously, it was `HashMap.toMap`.
3. During (2), we need to change the following to keep the original logic using `CaseInsensitiveMap.++`.
```scala
- val params = extraOptions.toMap ++ connectionProperties.asScala.toMap
+ val params = extraOptions ++ connectionProperties.asScala
```
4. Additionally, use `.toMap` in the following because `dsOptions.asCaseSensitiveMap()` is used later.
```scala
- val options = sessionOptions ++ extraOptions
+ val options = sessionOptions.filterKeys(!extraOptions.contains(_)) ++ extraOptions.toMap
val dsOptions = new CaseInsensitiveStringMap(options.asJava)
```
### Why are the changes needed?
`extraOptions.toMap` is used in several places (e.g. `DataFrameReader`) to hand over `Map[String, T]`. In this case, `CaseInsensitiveMap[T] private (val originalMap: Map[String, T])` had better return `originalMap`.
### Does this PR introduce _any_ user-facing change?
No.
### How was this patch tested?
Pass the Jenkins or GitHub Action with the existing tests and newly add test case at `JDBCSuite`.
Closes#29191 from dongjoon-hyun/SPARK-32364-3.
Authored-by: Dongjoon Hyun <dongjoon@apache.org>
Signed-off-by: Dongjoon Hyun <dongjoon@apache.org>
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