Use different threshold for (auto)scroll in output

Allow, in particular to switch to scolling for longer input (or disable
it) by still keeping the possibility to manually toggle the output to
scroll.
improve js documentation
rename scroll_threshold, add minimum_ prefix
document undefined behavior for 0 parameter
set autoscroll default back to 100
Merge pull request #3369 from Carreau/auto-scroll

Use different treshold for (auto)scroll in output

Use different treshold for (auto)scroll in output
Allow, in particular to switch to scolling for longer input (or disable
it) by still keeping the possibility to manually toggle the output to
scroll.

cf jsdoc, 
OutputArea.auto_scroll_threshold
and
OutputArea.minimum_scroll_threshold

OutputArea.auto_scroll_threshold < 0 will prevent auto_scroll from ever happening.