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
Depends. If one is aware of the meaning of section numbers, that "(5)" is very obviously suggesting that there is a file format named "crontab" which is documented. It's also pretty reasonable to suppose that the command and the file format of the same name are related.
A novice might miss the convention and the connection. Man pages are not quite novice material.
I can't count the number of times that following the trail laid out by 'SEE ALSO' sections a step or three has lead me to the exact thing that I never knew I needed to be using. Chasing those sections down is almost always well worth the three to ten minutes spent.
And, like, if one is expecting a man page to cover in detail everything even vaguely related to what it documents, and one doesn't feel one has ten minutes to spend reading things that people thought were important to bring to your attention... well, I guess one could go ask an LLM to slop out some related words. That'll probably take less than ten minutes, though correctness is not at all guaranteed.