bug#48019: 28.0.50; Error signaled by documentation function

[Juri Linkov]

Using help commands to get docstrings from the etc/DOC file
raises an error, e.g. on typing 'C-h m':

  Debugger entered--Lisp error: (wrong-type-argument listp t)
    documentation(auto-composition-mode)
    describe-mode()
    funcall-interactively(describe-mode)
    call-interactively(describe-mode nil nil)
    command-execute(describe-mode)

This is a recent regression.