bug#42707: Mention antidote to accidental k in dired

[積丹尼 Dan Jacobson]

   k runs the command dired-do-kill-lines (found in dired-mode-map),
   which is an interactive autoloaded compiled Lisp function in
   ‘dired-aux.el’.

   It is bound to k.

   (dired-do-kill-lines &optional ARG FMT)

   Kill all marked lines (not the files).
   With a prefix argument, kill that many lines starting with the current line.
   (A negative argument kills backward.)
   If you use this command with a prefix argument to kill the line
   for a file that is a directory, which you have inserted in the
   Dired buffer as a subdirectory, then it deletes that subdirectory
   from the buffer as well.
   To kill an entire subdirectory (without killing its line in the
   parent directory), go to its directory header line and use this
   command with a prefix argument (the value does not matter).

That's great. Except many users will be reading this after accidentally
hitting k, and wondering what happened. So at the bottom mention the antidote.

"If you accidentally hit k, then do ... (Undo? Yank?) to recover."