I finally found some documentations, I post them here to be useful for others ;)
- Enter the command
repo help manifest
(from Edward Falk's comment) - Edit the file
.repo/repo/docs/manifest-format.txt
(from Bjarke Freund-Hansen's answer) - Go to https://gerrit.googlesource.com/git-repo/+/master/docs/manifest-format.txt (from David Fraser's comment)