Add KDocs for `values` APIs

[zaleslaw]

Add KDocs for values APIs

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

Note: values in this file has two different meanings depending on the receiver; the KDocs must make this distinction clear.

Scope

Non-deprecated APIs, grouped by the two meanings:

• Flatten to a sequence (DataFrame) — DataFrame.values(byRow, columns), DataFrame.values(byRows), DataFrame.valuesNotNull(byRow, columns), DataFrame.valuesNotNull(byRow).
• Aggregation — collect column values per group / pivot cell — the values(...) overloads on Grouped, ReducedGroupBy, Pivot, ReducedPivot, PivotGroupBy, ReducedPivotGroupBy (the selector, vararg String, and no-column variants).

Out of scope: all deprecated vararg columns: AnyColumnReference / vararg columns: KProperty<*> overloads, and the internal GROUP_BY_VALUES_EXCEPTION_MESSAGE constant.

KDocs should explain

• DataFrame.values (flatten): returns a lazy Sequence of all cell values of the selected columns. The byRow / byRows flag controls traversal order — column-by-column by default (false), row-by-row when true. valuesNotNull is the same but drops nulls (and returns a non-nullable element type).
• Aggregation values: for each group / pivot cell, collects the values of the selected columns (defaults to the remaining columns) into the aggregation result. Explain the parameters:
  • dropNA — exclude NA/null values;
  • distinct — keep only distinct values;
  • separate (pivot variants) — how multiple aggregated columns are laid out;
  • the columns selector — which columns' values are collected (remaining columns if omitted).
• The Reduced* variants: they collect values from the single reduced row per group/cell.
• Mention the DuplicateColumnPathInsertException wrapping: the aggregation variants may fail with a duplicate-column error (e.g. after updateGroups/filter/add on groups) and rethrow a more descriptive message.

Test coverage assessment

• There is no dedicated test file for values.kt. Optional follow-up: cover the byRow order for DataFrame.values, valuesNotNull null-dropping, and the dropNA / distinct / separate options for the aggregation variants.

Website coverage

• Documented at values.html / getValues.html; keep KDocs consistent with those pages.

Acceptance criteria

• All non-deprecated values / valuesNotNull overloads have concise KDocs.
• No documentation is added for the deprecated / @AccessApiOverload overloads.
• KDocs clearly separate the two meanings: DataFrame.values (flatten to a Sequence) vs the aggregation values (collect per group / pivot cell).
• KDocs cover byRow, dropNA, distinct, and separate where they apply.
• Docs are consistent with the site pages values.html / getValues.html.