See http://fcron.free.fr/ and http://fcron.free.fr/doc/en/. The up-to-date documentation is also available at https://yo8192.github.io/fcron/.
Alternatively, if you downloaded fcron as a tarball, see:
- readme.txt in directory doc/en/txt/
- or readme.html in directory doc/en/HTML/.
If you cloned the git repo, you first need to build the documentation. On a Debian based system:
$ sudo apt install -y docbook docbook-xsl docbook-xml docbook-utils manpages-dev
$ autoconf
$ ./configure # optionally, use: --without-sendmail
$ make updatedoc