Why do some bash built-ins (such as dirs, pushd or help) don't have manual entries?
They have. It is
man bash or
man builtins. Bash is the software so it has the manual where its commands are explained. We shouldn't have a manual for every command we can give in a software. E.g
ftp has some same commands like
cd ) or same command like a binary (
ls), that would be very bad to make a manual for all of these. And I didn't talked about other shell's builtins.
they are also documented in
The reason they don't have manpages like
man help is likely because of possible confusion with other shell's builtins or a command by the same name.
I generally find this to be adequate. Both the manpages above are rather long and hard to navigate.