doc: remove custom callouts format

The code to render callouts for manpages comes from 17 years ago: 776e994af5 (Properly render asciidoc "callouts" in git man pages., 2006-04-28), and it was needed back then, but DocBook Stylesheets added support for that in 2008 [1], since 1.74.0 it hasn't been necessary. What's worse: the format of the upstream callouts is much nicer than our hacked version. Compare this: $ git diff (1) $ git diff --cached (2) $ git diff HEAD (3) 1. Changes in the working tree not yet staged for the next commit. 2. Changes between the index and your last commit; what you would be committing if you run git commit without -a option. 3. Changes in the working tree since your last commit; what you would be committing if you run git commit -a To this: $ git diff (1) $ git diff --cached (2) $ git diff HEAD (3) 1. Changes in the working tree not yet staged for the next commit. 2. Changes between the index and your last commit; what you would be committing if you run git commit without -a option. 3. Changes in the working tree since your last commit; what you would be committing if you run git commit -a Let's drop our unnecessary inferior custom format and use the official one. [1] https://sourceforge.net/p/docbook/code/7842/ Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>