Robust print CSS and clarified language

IMO the current version of this page can be confusing for people who aren't familiar with print styles and/or their CMS. Many people may simply copy and paste the code as it is written which will hide the uNav all of the time, not just during printing. It has already been copied erroneously at least [once](https://www.yammer.com/itsexternalcollaborations/#/Threads/show?threadId=463683300) so I took a stab at clarification.

I put the code inside a `@media print { }` query to ensure it only affects prints. This works just fine whether an implementer pastes the code into their regular "screen" CSS file or a print-specific style sheet. Even if the code is loaded in a file using `<link rel="stylesheet" media="print">` it will work as intended (only affecting print) despite the apparent redundancy. Therefore, I think we should use this more foolproof version to account for implementers putting the code in the "wrong" place. (Or even lacking access to a print style sheet.)

I also reworded the language to try and clarify the difference between hiding the uNav *contents* (the links, logo, etc) and hiding the uNav *container*. On Yammer I saw confusion about this.