Add a NOTE for "git commit --amend" to emphasize replacement of old commit.

While it might be obvious from that section as to how amending works,
it doesn't hurt to truly drive home the mechanics of amending.

Author: robertpjday

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

@@ -31,6 +31,14 @@ $ git commit --amend
 
 You end up with a single commit – the second commit replaces the results of the first.
 
+[NOTE]
+====
+It's important to understand that when you're amending your last commit, you're not so much fixing it as _replacing_ it entirely with a new, improved commit that pushes the old commit out of the way and puts the new commit in its place.
+Effectively, it's as if the previous commit never happened, and it won't show up in your repository history.
+
+The obvious value to amending commits is to make minor improvements to your last commit, without cluttering your repository history with commit messages of the form, ``Oops, forgot to add a file'' or ``"Darn, fixing a typo in last commit''.
+====
+
 [[_unstaging]]
 ==== Unstaging a Staged File