Add changelog for backports

docs/devel/changelog_staging.md

@@ -4,3 +4,7 @@
 <!-- List character: dash (-) -->
 
 # Changelog for next release
+*Backported bug fixes / corrections from current development branch to preferably leave v4 in a clean state.*
+- Fixed blunder in `PdfImage.extract()` producing an incorrect output path for prefixes containing a dot. In the `extract-images` CLI, this caused all output images of a type to be written to the same path for a document containing a non-extension dot in the filename.
+- XFA / rendering CLI: Fixed incorrect recognition of document length. `pdf.init_forms()` must be called before `len(pdf)`.
+- Made `get_text_range()` allocation adapt to pdfium version, as `FPDFText_GetText()` has been reverted to UCS-2. (See v4.28 changelog for background.)