Add KDocs for the remaining column-type predicates in `DataColumnType.kt`

Add KDocs for the remaining column-type predicates in DataColumnType.kt

The file core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/api/DataColumnType.kt is partially documented. Add concise KDocs to the predicates that still lack them.

Already documented (leave as-is)

isNumber, isMixedNumber, isPrimitiveNumber, isPrimitiveOrMixedNumber, valuesAreComparable. (isComparable is deprecated with ERROR level — out of scope.)

Scope — not documented

• AnyCol.isColumnGroup()
• AnyCol.isFrameColumn()
• AnyCol.isValueColumn()
• AnyCol.isSubtypeOf(type: KType)
• AnyCol.isSubtypeOf<T>() (reified)
• AnyCol.isType<T>() (reified)
• AnyCol.isList()

KDocs should explain

• The column-kind checks (isColumnGroup / isFrameColumn / isValueColumn): each returns true when the column is of that kind (ColumnKind.Group / Frame / Value). Mention that they carry a Kotlin contract — a true result smart-casts the receiver to ColumnGroup<*> / FrameColumn<*> / ValueColumn<*>.
• isSubtypeOf(type) / isSubtypeOf<T>(): returns true when the column's element type is a subtype of the given type, with nullability respected (a nullable column is not a subtype of a non-nullable type).
• isType<T>(): returns true only when the column's element type is exactly T (no subtyping) — contrast with isSubtypeOf.
• isList(): returns true when the column's element type class is List.

Acceptance criteria

• All seven predicates listed under Scope have concise KDocs; the already-documented predicates are preserved.
• The kind-check KDocs note the smart-cast contract.
• The KDocs make the isSubtypeOf (subtype, nullability-aware) vs isType (exact type) distinction explicit.
• No documentation is added for the deprecated isComparable.

Start in core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/api/DataColumnType.kt and review the existing KDocs for the documented predicates. Add concise KDocs for the seven listed predicates, including the kind-check smart-cast contracts and the nullability-aware subtype versus exact-type distinction. Done means all seven are documented, existing KDocs are unchanged, and isComparable remains undocumented.