[Connection Settings] Update strings for current bridges

I added a new string below the "Your Current Bridges" heading to help explain what's going on when the user sees multiple bridgelines:

You can keep one or more bridges saved, and Tor will choose which one to use when you connect to the Tor Network. If it fails, Tor will automatically switch to another.

I think that's vaguely accurate – right?

I also shortened the string within the bridge card itself a little:

Share this bridge using the QR code or by copying its address:

Here's how it looks in practice (although we should comment out the Learn mores within the Current Bridges section and bridge cards until the tb-manual is updated with destinations for them):