(try_scrolling): Set scroll_max to max of scroll_* args

(try_scrolling): Set scroll_max to max of scroll_* args
so setting scroll-step to 1 doesn't defeat scroll-conservatively.
Set amount_to_scroll to max of dx and scroll_step so that
scroll-conservatively doesn't defeat scroll-step>1.
(syms_of_xdisp): Add a hint in scroll-step's docstring to use
scroll-conservatively for line-at-a-time scrolling.