Docs for #2228

Add docs that when completions are object they can have a from: and to:
that will be used by pick.

I took the liberty of adding a CSS fix for small screen:

Make sidebar in manual scrollable for small screen especially usefull
when `Programming API` is unfolded

Seem to look nice on Chrome and Firefox Osx, Scrollbar do not appear,
(except when scrolling on Chrome/safari) but should maybe be tested on linux.