Fix info-lookup docs and "C-h S" in major-modes with no manuals

Fix info-lookup docs and "C-h S" in major-modes with no manuals

* lisp/info-look.el (info-lookup-alist, info-lookup-add-help)
(info-lookup-symbol, info-lookup-file, info-lookup)
(info-lookup-interactive-arguments): Clarify and expand the doc
strings.
(info-lookup-change-mode): Topic values can have their `:mode' as
a cons cell, not just a mode's symbol; support that.