Care to really quantify that, based on real experience? Here's my ha'p'orth.
Linux kernel: You get the source and quite a lot of info on what each option does in menuconfig or whatever.
Mailer daemons (for example): Postfix, Exim and Sendmail are very well documented, have excellent mailing lists. Masses of examples across the web and shed loads of forums and postings
Samba, BIND, KDE, Gnome, Apache, nginx, HA Proxy, Elastic Search, Hadoop, Postgres, MariaDB, libvirtd, Xorg, NetworkManager, FreeCAD, LibreCAD, LibreOffice, OpenOffice, Evolution, Krita, Scribus
Do you really think that a distro needs to do anymore than note down what they do that is "different"?