Document keyboard shortcut for emoji reactions

### Your use case

#### What would you like to do?

Under Settings->Keyboard I'd like to see how to add a reaction to the last post using only my keyboard.

From looking at the following pull request...

- https://github.com/matrix-org/matrix-react-sdk/pull/5425

... I learned that I can type `+:smile:` (tab complete is supported) and it works great! The next step is to add it under the existing list of shortcuts, which looks like this as of Element 1.11.22:

#### Why would you like to do it?

Because I'm tired of reaching for my mouse every time I'd like to add a reaction to a message. I use a keyboard shortcut for this all the time in Slack.

#### How would you like to achieve it?

I assume this is just a matter of editing the file or files that power the screenshot above. I took a quick look, but I don't know where they are. I could maybe try to make a pull request if someone points them out, but I've never set up a dev environment for Element, so if someone else can do this quickly, that's fine with me!

### Have you considered any alternatives?

I suppose I could maintain my own list of shortcuts.

### Additional context

Element and Matrix are awesome! Thank you! ❤️ 🎉 🚀