Add KDocs for `unfold` APIs

[zaleslaw]

Add KDocs for unfold APIs

The file core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/api/unfold.kt has no KDocs at all. Please add concise KDocs for the non-deprecated APIs, excluding deprecated / @AccessApiOverload overloads.

Context / relation to #677

Issue #677 ("Add tests for unfold") raised two things:

• Tests — this is now largely done: core/src/test/.../api/unfold.kt (UnfoldTests) covers the basic object-to-columns unfold, maxDepth deep unfolding, "keep value type" (value columns stay untouched), and pair-of-DataFrame / pair-of-@DataSchema structures. So tests are not the primary gap anymore.
• The isPrimitive() → isValueType() rename proposed there appears already resolved: the concept now lives as the internal KClass<*>.isValueType in impl/api/toDataFrame.kt.

What remains missing is documentation on the API itself — hence this KDocs ticket.

Scope

Non-deprecated APIs in unfold.kt:

• DataColumn<T>.unfold(vararg roots: KCallable<*>, maxDepth: Int = 0)
• DataFrame<T>.unfold(vararg roots: KCallable<*>, maxDepth: Int = 0, columns: ColumnsSelector<T, *>)
• DataFrame<T>.unfold(vararg columns: String)

Out of scope: the deprecated vararg columns: AnyColumnReference / vararg columns: KProperty<*> overloads.

KDocs should explain

• What unfold does: it is the in-DataFrame equivalent of an Iterable<>.toDataFrame() conversion — it takes the selected value column(s) holding objects (or Iterable / DataFrame values) and expands them in place into ColumnGroups / FrameColumns by reading their properties, returning a new DataFrame/column with those columns replaced. (See unfold.html.)
• That columns holding plain value types are left unchanged (as asserted by the keep value type test).
• The roots and maxDepth parameters: which properties are traversed and how deep nested objects are expanded (maxDepth = 0 = shallow).
• The columns selector / vararg String overload: which columns get unfolded.
• The relationship to toDataFrame (same semantics, applied inside a frame) — and that it is implemented on top of replace(columns).with { ... }.

Acceptance criteria

• All three non-deprecated unfold overloads have concise KDocs.
• No documentation is added for the deprecated / @AccessApiOverload overloads.
• KDocs state the toDataFrame-equivalent semantics, the value-type-kept-as-is behavior, and the meaning of roots / maxDepth.
• Docs are consistent with the site page unfold.html.
• (Optional follow-up, per Add tests for unfold #677) if desired, add one test asserting unfold stays consistent with the Iterable<>.toDataFrame() overloads, which Add tests for unfold #677 noted was never explicitly verified.