update on extlinux docs

[Matthew Miller]

So, I don't think syslinux-extlinux is going to make it onto the install
media, which means that the extlinux option will only be available for net
installs where the "Everything" tree is available. That should probably go
in the extlinux section.

-- 
Matthew Miller  ☁☁☁  Fedora Cloud Architect  ☁☁☁  <mattdm(a)fedoraproject.org&gt;