Why does rdoc sometimes not install?

So on Prawn’s website they rub it in that we should use the local rdoc
as
they dont guarantee their web docs to be up to date. So I start my
gemserver
and I see that none of the prawn gems have installed rdoc. I have seen
this
occur on other gems to but not all. Is there something I need to do to
get
the rdoc? I did not install using --no-rdoc, etc.

prawn 0.8.4 [rdoc] [www] http://wiki.github.com/sandal/prawn -
depends
on prawn-core http://localhost:8808/#prawn-core,
prawn-layouthttp://localhost:8808/#prawn-layout,
prawn-security http://localhost:8808/#prawn-security. A fast and
nimble
PDF generator for Ruby