/usr/share/irssi/help/quit is in irssi 0.8.15-5ubuntu3.6.
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 | QUIT %|[<message>]
This ends your IRC session. If a quit message is supplied, it will be displayed to anyone else on any channel you were on before quitting. If one isn't specified, the text "Leaving" is used.
/EXIT does the same.
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