Tooltips in the Notebooks chooser

From the development repository at e819bc4.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

docs/implementation.md

@@ -177,7 +177,10 @@ a page of another session in the notebook.  Nothing is copied if the
 page is blank or the same page is already kept in that notebook; a
 page loaded from a notebook and left unchanged is therefore not kept
 again, although the file names it as a new page.  A kept page or a
-whole notebook can be deleted from the Notebooks chooser; the local
+whole notebook can be deleted from the Notebooks chooser, whose
+tooltips describe each page (from the page file itself, which the
+chooser fetches for the picture) and each notebook (from the listing,
+which names the pages and says when each was last written); the local
 program moves it to the system's Trash (the freedesktop Trash on
 Linux, with the info file the file manager wants for putting it back;
 `~/.Trash` on macOS) unless `--trash` says otherwise, and the