Bug#637249: document undocumented prejudice against experimental

[jidanni@jidanni.org]

Package: apt
Version: 0.8.15.5
Severity: wishlist

One thought one could comment out

//APT::Default-Release "experimental"; //instead just list it first in sources.list

but in fact the "cards are still stacked" against experimental.
There is some behinds the scenes _undocumented built in prejudice_
against it.

apt-config dump doesn't show it.
man apt_preferences says a tiny bit

       priority 1
           to the versions coming from archives which in their Release files
           are marked as "NotAutomatic: yes" but not as "ButAutomaticUpgrades:
           yes" like the debian experimental archive.

probably unrelated.