The second line you quoted tells you how.
cd $(mktemp -d)
git init
echo bar > file.txt
git add file.txt
git commit -m "init"
echo foo_bar_baz > file.txt
# select only "foo_" in magit-status and press "s"
git diff --cached
diff --git a/file.txt b/file.txt
index 5716ca5..2fd000c 100644
--- a/file.txt
+++ b/file.txt
@@ -1 +1,2 @@
bar
+foo_bar_baz
diff --git a/file.txt b/file.txt
index 5716ca5..2fd000c 100644
--- a/file.txt
+++ b/file.txt
@@ -1 +1,2 @@
-bar
+foo_bar
I just updated magit, too. It seems to have been published 3 days ago.
I could swear I remember this working in the past, but I can find no hint of it in a few quick searches, so I'm likely mistaken.
I'm sorry for the misinformation. Thanks for saying something.