Bash, at least, is more helpful these days.

    $help
    GNU bash, version 5.0.17(1)-release (x86_64-pc-linux-gnu)
    These shell commands are defined internally.  Type `help' to see this list.
    Type `help name' to find out more about the function `name'.
    Use `info bash' to find out more about the shell in general.
    Use `man -k' or `info' to find out more about commands not in this list.

That last sentence in particular.