Add warning about git restore --staged -- <file>

Warn readers not to throw away their local work without meaning to.
This is basically a copy/paste and adaptation of the warning we give about `git checkout -- <file>`.

Author: HonkingGoose

book/02-git-basics/sections/undoing.asc

@@ -225,4 +225,11 @@ Changes to be committed:
   (use "git restore --staged <file>..." to unstage)
 	renamed:    README.md -> README
 
-----
+----
+
+[IMPORTANT]
+=====
+It's important to understand that `git restore --staged <file>` is a dangerous command.
+Any local changes you made to that file are gone -- Git just replaced that file with the most recently-committed version.
+Don't ever use this command unless you absolutely know that you don't want those unsaved local changes.
+=====