Change shortcut choser dialog to be more explicit

Following an initial prompt, an additional detailed explanation appears below.

3-button screenshot: https://x20web.corp.google.com/users/jo/jonesriley/bugs/327670156/screencap_3button.png

Gesture screenshot: https://x20web.corp.google.com/users/jo/jonesriley/bugs/327670156/screencap_gesture.png

Test: N/A
Flag: EXEMPT xml-based change
Bug: 327670156
Change-Id: I80cee4630e62afac1a14b4014c2f9fb91141d7bf