The man page tells that "Explanation: " would work for commenting package sections (and "//" does appear to work the same), but it requires a Package header then anyway (otherwise it results in "E: Invalid record in the preferences file /etc/apt/preferences, no Package header").
This has regressed:
% cat /etc/apt/ preferences
#test
% apt-get source apt preferences (1)
Reading package lists... Done
E: Unable to parse package file /etc/apt/
The man page tells that "Explanation: " would work for commenting package sections (and "//" does appear to work the same), but it requires a Package header then anyway (otherwise it results in "E: Invalid record in the preferences file /etc/apt/ preferences, no Package header").