Add minimal KDocs for `print` APIs

# Add minimal KDocs for `print` APIs

## Motivation

`core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/api/print.kt` contains several public `print()` functions with no KDoc.

Most of them are intentionally thin convenience wrappers around `println(...)` or `println(renderToString(...))`, but this is not visible from the API docs. A minimal KDoc would help users understand when to call `print()` directly and when to use the underlying string-rendering APIs instead.

In particular, `DataFrame.print(...)` exposes formatting parameters and delegates to `renderToString(...)`, so its KDoc should clarify that it prints the rendered table to standard output and is mainly intended for quick interactive/debug output.

## Scope

Add concise KDocs for public non-deprecated functions in:

`core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/api/print.kt`

Cover:

- `DataColumn.print()`
- `DataRow.print()`
- `DataFrame.print(...)`
- `GroupBy.print()`
- `DataFrameSchema.print()`
- `CodeString.print()`

Deprecated hidden overloads do not need new KDocs unless required by the project’s documentation policy.

## Acceptance Criteria

- Public non-deprecated `print` APIs have minimal KDocs.
- KDocs state that these functions print to standard output.
- `DataFrame.print(...)` KDoc mentions that it renders via `renderToString(...)`.
- `DataFrame.print(...)` parameters are documented briefly enough to explain when to use them.
- KDocs clarify that `print()` is a convenience/debug/interactive output API, not a data transformation.
- KDocs point users to `renderToString(...)` when they need the rendered table as a `String` instead of printing it.

Start in core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/api/print.kt and review the six listed public, non-deprecated print APIs and their renderToString(...) relationship. Add concise KDocs describing standard-output printing, convenience or interactive use, and the DataFrame.print(...) parameters. Done means all listed APIs are documented and users are directed to renderToString(...) when they need a String.

Written by the indexing model from the issue text.