Usability papercuts on keyboard shortcut help page

As a new user who was recently viewing the Notebook help page on keyboard shortcuts, I encountered two usability papercuts:
- The shortcuts are displayed with capital letters, but they are only activated by lowercase letters. At least, that's how things are on my Linux/Firefox setup. At first I thought nothing was working because I was typing (say) "A" instead of plain lowercase "a".
- The meaning of the comma syntax like "D, D" for "delete cell" is not at all clear. It took me a while to figure out that it meant I needed to press the keys twice.

Start from the Notebook keyboard-shortcut help page and reproduce the examples in Linux/Firefox. Clarify lowercase activation and explain comma-separated repeated shortcuts such as “D, D”; done when the displayed guidance matches behavior and both ambiguities are resolved.

Written by the indexing model from the issue text.