changelog: explicitly mention previous _flatten()

docs/devel/changelog_staging.md

@@ -23,7 +23,7 @@
 *Improvements and new features*
 - Added `PdfPosConv` helper and `PdfBitmap.get_posconv(page)` for bidirectional translation between page and bitmap coordinates.
 - Added `PdfObject.get_quad_points()` to get the corner points of an image or text object.
-- Exposed `PdfPage.flatten()` (previously non-public helper), after having found out how to correctly use it. Added check and updated docs accordingly.
+- Exposed `PdfPage.flatten()` (previously semi-private `_flatten()`), after having found out how to correctly use it. Added check and updated docs accordingly.
 - Added context manager support to `PdfDocument`, so it can be used in a `with`-statement, because opening from a file path binds a file descriptor, which should be released safely and as soon as possible, given OS limits on the number of open FDs.
 - If document loading failed, `err_code` is now assigned to the `PdfiumError` instance so callers may programatticaly handle the error subtype.
 - Corrected some null pointer checks: we have to use `bool(ptr)` rather than `ptr is None`.