cron(8) mentions: "The cron utility also searches for /etc/crontab which is in a different format (see crontab(5))"
crontab(5) then mentions, somewhere hidden inside the wall of text, the specifics of the different format: "Each line has five time and date fields, followed by a user name (with optional ``:<group>'' and ``/<login-class>'' suffixes) if this is the system crontab file, followed by a command."
So the specifics of the format of /etc/crontab are mentioned in a subclause in the middle of crontab(5). What this extra field _does_ is not specified (one would assume it's being used to su to the user, but there's no mention of what effect either :group, nor /login-class would do on top of the user's native group or login class).
I suggest moving the specifics of /etc/crontab's format into an own paragraph, complete with paragraph header so that when glancing over crontab(5) one will find it easily and quickly, along with _detailed_ information about what this extra field does, also in conjunction with cron(8)'s flags, see below.
cron(8) mentions: "-J rootjitter Enable time jitter for superuser jobs. The same as -j except that it will affect jobs run by the superuser only."
- Does this apply to the "system crontab" only? Or to the root's USER crontab only? Or to any *tab* run by root (i.e., /etc/crontab, root's crontab, toor's crontab, and any other uid=0 user's crontab?) What about the aforementioned sixth-user-name-field jobs in /etc/crontab? Does it apply -j or -J?
I suggest properly documenting this in cron(8), with a reminder in crontab(5).
/etc/crontab mentions: "/etc/crontab - root's crontab for FreeBSD". Well, it is NOT root's crontab. (A) It has support for a special field that users' crontabs don't, (B) root's "crontab -e" will NOT pull up /etc/crontab, but root's own crontab.
I suggest s/root's/system/;s/D/D - cf. crontab(5)/