Toggle highlighting of the current line (Hl-Line mode).
With a prefix argument ARG, enable Hl-Line mode if ARG is
positive, and disable it otherwise. If called from Lisp, enable
the mode if ARG is omitted or nil.
Hl-Line mode is a buffer-local minor mode. If
`hl-line-sticky-flag' is non-nil, Hl-Line mode highlights the
line about the buffer's point in all windows. Caveat: the
buffer's point might be different from the point of a
non-selected window. Hl-Line mode uses the function
`hl-line-highlight' on `post-command-hook' in this case.
When `hl-line-sticky-flag' is nil, Hl-Line mode highlights the
line about point in the selected window only. In this case, it
uses the function `hl-line-unhighlight' on `pre-command-hook' in
addition to `hl-line-highlight' on `post-command-hook'. (fn &optional ARG)