The zopflipng tool is really great for shrinking PNG files, but it, unfortunately, lacks a manpage. One could be generated from the output of zopflipng itself, which would make Debian better documented and satisfy people that type "man foo" by muscle memory (like me) for any new commands to inspect their options. Thanks, Rogério Brito.
I created a man page for zopfli with the help of help2man. zopfli.1 is the rendered man page generate-man-page.sh contains the help2man command with its options for creating zopfli.1 zopfli-include.1 is included by generate-man-page.sh