doc(python_code_splitter): add comments for the identity preservation logic

Author: GovindhKishore

haystack/components/preprocessors/python_code_splitter.py

@@ -556,6 +556,9 @@ def _secondary_split(self, unit: _CodeUnit, parent_doc: Document) -> list[Docume
             meta["secondary_split"] = True
             meta["secondary_split_index"] = idx
             meta["secondary_split_total"] = len(intermediate)
+
+            # Prepend the unit's qualified name as a comment to non-first pieces,
+            # since only the first piece naturally retains the def/class signature line.
             content = piece.content or "" if idx == 0 else f"# {qualified_name}\n{piece.content or ''}"
             results.append(Document(content=content, meta=meta))
         return results