Improve documentation for "other window" scrolling

Improve documentation for "other window" scrolling

* doc/emacs/windows.texi (Other Window):
* doc/lispref/windows.texi (Textual Scrolling):
Document scroll-other-window-down.
* doc/lispref/minibuf.texi (Minibuffer Misc):
Cross-reference minibuffer-scroll-window with Textual Scrolling.
* src/window.c (Fother_window_for_scrolling):
Clarify how "other window" is determined in docstring.
(Fscroll_other_window): Simplify docstring, pointing to that of
Fother_window_for_scrolling. (bug#30207)