In fact, the only reference to crontab(5) is in the SEE ALSO section (on my version anyway), but that doesn't say why you might want to see crontab(5), just that it exists. That is spectacularly useless

That is incredibly stupid. A documentation system designed by someone who doesn't understand how people use documentation.

If man was designed by someone with any taste at all it would at least give you a menu to select (1) crontab command, (5) crontab file format. Maybe we need a rewrite in Rust to fix that.

man -k crontab is the real trick here. shows both sections so you don't have to already know the number exists.