Merge pull request #321 from Kotlin/type-argument-docs

Provide extra pointers to operations with DataFrame type argument

Author: koperagen

docs/StardustDocs/topics/adjustSchema.md

@@ -2,7 +2,11 @@
 
 [`DataFrame`](DataFrame.md) interface has type argument `T` that doesn't affect contents of [`DataFrame`](DataFrame.md), 
 but marks [`DataFrame`](DataFrame.md) with a type that represents data schema that this [`DataFrame`](DataFrame.md) is supposed to have.
-This argument is used to generate [extension properties](extensionPropertiesApi.md) for typed data access. 
+This argument is used to generate [extension properties](extensionPropertiesApi.md) for typed data access.
+
+Another place where this argument has a special role is in [interop with data classes](collectionsInterop.md#interop-with-data-classes):
+* `List<T>` -> `DataFrame<T>`: [toDataFrame](createDataFrame.md#todataframe)
+* `DataFrame<T>` -> `List<T>`: [toList](toList.md)
 
 Actual data in [`DataFrame`](DataFrame.md) may diverge from compile-time schema marker `T` due to dynamic nature of data inside [`DataFrame`](DataFrame.md). 
 However, at some points of code you may know exactly what [`DataFrame`](DataFrame.md) schema is expected.