@b0rk In the times when many gnu tools moved to info pages, there were many inconsistencies and missing details where "--help" output, man pages and info docs were out of sync, and often distributions would in turn miss out on things like sed(1) not going into regexps per se, but pointing to another package like perlre to explain the format of basic and extended regexps (and difference between posix regexps and the one gnu sed supports). So while the latter is a fault(?) of the distribution packaging if the SEE ALSO links are not there, it still leaves you as the user without the needed docs in man format. While on BSDs the base tools can and do refer to other installed base tools but unless you deliberately strip out things, the links would work out, and you would be able to read about re_format(7) from the base tools that do support regexps.
But just to take one example from the "ip" command that replaced "ifconfig", on Ubuntu 24.04, the synopsis lists a certain amount of commands and options. Still, the options below contain -echo which is not in the synopsis, even though the synopsis does contain "long" options already. So at some point, someone added -echo to have the kernel tell you what got applied, it got its line in the lower part of the manpage, but synopsis doesn't mention it. Also, "ip --help" doesn't list it either.