(It's generally simplest when executables are as self-contained as
possible. I've long wondered why programs don't store their manual pages
in a special section of their ELF executable. It wouldn't be hard and
would solve the common problem of missing or out-of-sync manual pages.)
I'm ambivalent.
In this multilingual era, having manpages (in dozens of languages) within executable seems too much.
I definitely prefer having basic instructions within the executable; and also if it can point to a canonical URI
of documentation, it's not too much to seamlessly fetch the document from there. So it's not so much about
begin in one binary, but having consistent and robust link between related resources.