Improve ASCII-armored keys export documentation.

[TrevCan]

Regarding the second option to exporting gpg keys using the ascii-armored format, I was perplexed by the fact that there were two QR codes for each key. See https://github.com/yishilin14/asc-key-to-qr-code-gif/issues/4 . I understand that a .gif is an animated image but at the time I was using a static image viewer so I was only seeing the first frame of the gif. I think apart from this, it should also be added explicitly (and within the app) that the QR code scanner expects two QR codes for one scan (right?, please correct me if I'm wrong).