* lisp/mwheel.el (mouse-wheel-scroll-amount): Change 'shift' default value
from 5 to 'hscroll'. Add new option "Scroll horizontally" for 'hscroll'.
(mwheel-scroll): Handle value 'hscroll' and call mwheel-scroll-left-function
or mwheel-scroll-right-function.
* doc/emacs/frames.texi (Mouse Commands): Update for horizontal scrolling
with Shift modifier.
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