tools/magit: add magit-diff-refine-hunk to readme

modules/tools/magit/README.org

@@ -13,6 +13,7 @@
 - [[#features][Features]]
 - [[#configuration][Configuration]]
   - [[#enable-gravatars][Enable Gravatars]]
+  - [[#enable-granular-diff-highlights-for-all-hunks][Enable granular diff-highlights for all hunks]]
 - [[#troubleshooting][Troubleshooting]]
 
 * Description
@@ -62,5 +63,16 @@ This will enable gravatars when viewing commits. The service used by default is
 (setq magit-revision-show-gravatars '("^Author:     " . "^Commit:     "))
 #+END_SRC
 
+** Enable granular diff-highlights for all hunks
+
+By default, changes are highlighted *linewise* for all but the selected hunk. This
+has [[https://magit.vc/manual/magit/Performance.html][performance reasons]]. You can enable character-wise highlights for all
+visible hunks with:
+
+#+BEGIN_SRC emacs-lisp
+(after! magit
+  (setq magit-diff-refine-hunk 'all))
+#+END_SRC
+
 * TODO Troubleshooting
 # Common issues and their solution, or places to look for help.