bug#61465: so-long-commentary: say how to prepare against long line output

[Dan Jacobson]

(so-long-commentary)
should mention how to guard against
M-! shell-command "cat long_line_file.txt", and
M-x compile "cat long_line_file.txt"
in addition to just dealing with
M-x find-file long_line_file.txt .

emacs-version "28.2"