Expand replaces Widen: fill the window in both directions

From the development repository at 3f1da52.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/implementation.md

@@ -113,7 +113,8 @@ from the host (or from an allowed guest, for pages and images).
 ## Pages
 
 Page coordinates are in units of 0.1 mm.  A blank page is an A4 sheet
-in landscape, 2970 by 2100 units.  "Widen" makes the sheet wider, and
+in landscape, 2970 by 2100 units.  "Expand" makes the sheet fill the
+window in both directions, and
 "Shrink" makes it the bounding box of everything on the page plus the
 greater of the two margins, exactly, on every side; both are changes to the page like adding
 an image, carried in the page descriptor (its `left` and `top` edges and