Part of #868: docs for Save As .ai legacy version 7 (artboard picker, Transparent Images)

docs/gaps.md

@@ -81,8 +81,12 @@ from G14 and the budget run of G15: **~170–270 h** one agent, ~50–80 h wall
   of 14), guides and non-printing construction
   lines (#779), open speed on large files (#758), legacy type in `.ai` files in PostScript form (those without
   their prolog open since #1027), live effects, brushes and symbols as live objects (#637; pattern fills and global colours are read since #1025, placed (linked) files since #1264). Writing Illustrator's own editing data is out of scope
-  (undocumented); Save As `.ai` stays PDF-compatible, and Illustrator 2018 opens such a file without its layers and
-  groups (#868).
+  (undocumented): a PDF-compatible `.ai` opens in Illustrator 2018 without its layers and groups (#868). Save As
+  `.ai` › Version: Legacy (version 7) writes the PostScript form from the published version 7 specification
+  instead, keeping layers (names, order, visibility, lock), groups, compound paths, clipping groups, gradients and
+  images, one artboard per file (an artboard picker chooses which); transparency, type, live effects and object
+  names don't survive it (transparent images are flattened or omitted). It waits for testing by users who hand
+  files back.
 - **Evidence:** the issues above; 28 of the 350 issues are about `.ai`/EPS (about 22 people, the most common single
   format theme), 20 fixed, and the 8 open ones are about structure (layers, guides, speed, live objects), not missing
   art. No real-file `.ai` corpus (users' files may be used locally, never committed).