Add KDocs for `xs` (cross-section) APIs

[zaleslaw]

Add KDocs for xs APIs

The file core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/api/xs.kt has no KDocs at all. Please add concise KDocs for all four functions.

Scope

• DataFrame<T>.xs(vararg keyValues: Any?) — default-key-columns overload.
• DataFrame<T>.xs(vararg keyValues: C, keyColumns: ColumnsSelector<T, C>) — explicit key-columns overload.
• GroupBy<T, G>.xs(vararg keyValues: Any?) — default-key-columns overload.
• GroupBy<T, G>.xs(vararg keyValues: C, keyColumns: ColumnsSelector<T, C>) — explicit key-columns overload.

KDocs should explain

• What xs does (cross-section): returns the subset containing only the rows whose key columns match the given keyValues, in order — like fixing the leading key columns to specific values. For DataFrame it returns a filtered DataFrame<T>; for GroupBy it filters within the grouped structure and returns a GroupBy<T, G>.
• The keyValues → keyColumns positional matching: the i-th keyValues is matched against the i-th selected key column.
• The default key-column selection (the overloads without keyColumns): the first keyValues.size non-column-group columns at any depth (colsAtAnyDepth().filter { !it.isColumnGroup() }.take(keyValues.size)).
• That the matched key columns are typically dropped from the result (cross-section semantics) — verify against xsImpl and state the actual behavior.

Test coverage assessment

• DataFrame.xs has coverage (DataFrameTests.xs, plus usage in DataFrameTreeTests and samples/api/Access.kt).
• GroupBy.xs has no obvious dedicated test — optional follow-up to add one.

Website coverage

• Documented at xs.html; keep KDocs consistent with it.

Acceptance criteria

• All four xs overloads have concise KDocs.
• KDocs explain the cross-section semantics, the positional keyValues↔keyColumns matching, and the default key-column selection.
• KDocs distinguish the DataFrame result from the GroupBy result.
• Docs are consistent with the site page xs.html.

[zaleslaw]

Closed in #2052