Fixes
 - `install-latest-npm`: `npm` `v6.10` breaks on `node` `v6.0`, `v6.1`, and `v9.0`-`v9.2`
 - `nvm use`: improve help output with bare `nvm use` and no `.nvmrc`

Docs
 - Add description if instructions don't work; make install instructions more readable (#2112)