Add KDocs for `DataRowSchemaApi`

# Add KDocs for `DataRowSchemaApi`

The file `core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/api/DataRowSchemaApi.kt` has no KDocs. Add concise KDocs for its three public declarations.

### Scope

- `DataRowSchema` — the marker interface a user's data class implements to be usable as a typed row schema.
- `dataFrameOf(vararg rows: T)` — builds a `DataFrame` from row objects of a `DataRowSchema` type.
- `DataFrame.append(vararg rows: T)` — returns a new `DataFrame` with the given schema rows appended.

### KDocs should explain

- `DataRowSchema`: what implementing it enables (strongly-typed `dataFrameOf` / `append`), and that rows are converted via `toDataFrame()` reflection.
- `dataFrameOf`: each row object becomes one row; columns are derived from the schema's properties.
- `append`: it does not mutate — it returns a new frame (source + appended rows concatenated).

### Acceptance criteria

- All three declarations have concise KDocs.
- The KDocs note that `append` returns a new `DataFrame` (no in-place mutation) and that rows map to columns by the schema's properties.