Add more clarity what git restore and git checkout -- <file> does

[BPylypenko]

Which version of the book is affected?
both

Describe the bug:
The book said that git restore <file> or git checkout -- <file>:

Git just replaced that file with the most recently-committed version

but, as I see, it actually reverts to the most recently-staged version

Steps to reproduce:

1. Go to chapter 2.4
2. And pay attention to the IMPORTANT notes.

Expected behavior:
where it said the most recently-committed version should say the most recently-staged version or the last staged or committed version