STY: Tweak PdfWriter (#3337)

Author: j-t-1

pypdf/_writer.py

@@ -209,7 +209,7 @@ def __init__(
 
         self._info_obj: Optional[PdfObject]
         """The PDF files's document information dictionary,
-        the Info entry in the PDF file's trailer dictionary."""
+        defined by Info in the PDF file's trailer dictionary."""
 
         self._ID: Union[ArrayObject, None] = None
         """The PDF file identifier,
@@ -625,7 +625,7 @@ def insert_page(
         """
         assert self.flattened_pages is not None, "mypy"
         if index < 0:
-            index = len(self.flattened_pages) + index
+            index += len(self.flattened_pages)
         if index < 0:
             raise ValueError("Invalid index value")
         if index >= len(self.flattened_pages):
@@ -690,7 +690,7 @@ def insert_blank_page(
         """
         Insert a blank page to this PDF file and return it.
 
-        If no page size is specified, use the size of the last page.
+        If no page size is specified for a dimension, use the size of the last page.
 
         Args:
             width: The width of the new page expressed in default user