Say "page", not "sheet", everywhere a user sees it

From the development repository at 854009c.

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

docs/implementation.md

@@ -112,19 +112,20 @@ 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.  "Expand" makes the sheet fill the
+Page coordinates are in units of 0.1 mm.  A blank page is A4
+in landscape, 2970 by 2100 units.  "Expand" makes the page 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
 its size) with the images' revision raised.  Writing within 5 mm of an edge grows the page by
-5 mm on that side, counted from the sheet's edge.  An image is placed at its natural size, taken from
+5 mm on that side, counted from the page's edge.  An image is placed at its natural size, taken from
 the resolution recorded in the file, or 0.1 mm per pixel when there is
 none (the nominal 72 and 96 dpi that cameras and screenshots write count
 as none), centred across the page, 5 mm from the top or from the image
-above, the sheet widened equally on both sides if it is wider than the
-page; only one wider than a page can be, 2 m, is scaled down.  A page holds at most 12 images and 24 MB of image data.
+above, the page widened equally on both sides if the image is wider
+than it and lengthened downward if the image is taller; only one wider
+than a page can be, 2 m, is scaled down.  A page holds at most 12 images and 24 MB of image data.
 An image file over 2400 pixels on a side or 1.5 MB, or not PNG or JPEG,
 is redrawn at most 2400 pixels and stored as JPEG before it is shared;
 smaller PNG and JPEG files are shared byte for byte.