bug#32863: Unsatisfactory "definition" of "vertical scroll position" in Emacs lisp manual and doc string of window-vscroll

[Alan Mackenzie]

Hello, Emacs.

In the Emacs-26.1 Emacs Lisp manual, on page "Vertical Scrolling" there
is an ostensible definition for "vertical scroll position".

This "definition" says it "is a number, never less than zero.  It
specifies how far to raise the contents of the window."

What should be doing this raising?  When might it do this?  For what
purpose might the contents of the window be raised?

I find this "definition" totally obscure.  I can not make sense of it at
all.  Without understanding what "vertical scroll position" means, the
entire manual page is meaningless.

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I came to this manual page through not understanding the doc string for
the function window-vscroll.  This says just "Return the amount by which
WINDOW is scrolled vertically.".

_Is_ scrolled vertically.  What on earth does that mean?  What is the
zero point from which this scrolling is measured?  Does this "is" refer
to the current visible scrolling, or the intended scrolling after the
next redisplay?

This doc string needs clarification.

-- 
Alan Mackenzie (Nuremberg, Germany).