Add KDocs for `Join` and `JoinWith`

[zaleslaw]

https://github.com/Kotlin/dataframe/blob/master/core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/api/join.kt
https://github.com/Kotlin/dataframe/blob/master/core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/api/joinWith.kt

• Also, please check coverage for Joins and Joins DSL in Unit-test, test missed cases