@romainl has written a great summary of how to use Vim's documentation.
I think this should be copied into, or at the very least linked from, the site's official help pages. It's universally useful resource. (I, for one, would have probably saved a lot of time if I'd found out about
:helpgrep at any point in the past ten years of using Vim.)
It should also, if possible, be waved fairly obtrusively in front of users before they ask their first question on the site. (ISTR that some (all?) SE sites already have functionality along those lines.)