/usr/share/irssi/help/part is in irssi 0.8.19-1ubuntu1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 | %9Syntax:%9
PART %|[<channels>] [<message>]
%9Parameters:%9
The channels, separated by a comma, to leave and the message to advertise.
%9Description:%9
Leaves the given channels.
%9Examples:%9
/PART #irssi
/PART #freenode,#irssi
%9See also:%9 JOIN, KICK
|