[MRG] More documentation minor fixes

A PR I plan to keep open while I am browsing the documentation and bumping into minor things that can be improved.
> "guarantee the future" seems to win the google battle against "warrant the
> future". Also this link shows a few occurrences from the European parliament,
> which hopefully hires translators we can trust.

OK, point taken.

> Otherwise the same link has something like "to (help) ensure the
> future" if you like it better.

It seems to sound better to my ears, but whatever the european parliement
says is good!