Not always an option when you have to manually patch proftpd to fix its SSL mess .
And besides it's beyond the point. If there is outdated doc out there, it should either be fixed or removed.
That doc also has a prefix that works on FreeBSD, but not necessarily on Linux.
Regarding the install_user=ftp install_group=ftp bit, that's not related to any particular OS, but rather the shell you're using. You can take it out if you don't use a "Bourne-ish style shell": http://www.proftpd.org/docs/howto/Compiling.html