[version-control] Document how to disable diff-hl

Author: sunlin7

layers/+source-control/version-control/config.el

@@ -30,7 +30,7 @@
 
 (spacemacs|defc version-control-diff-tool 'diff-hl
   "Options are `diff-hl' (the preferred choice) or `git-gutter' to show
-version-control markers."
+version-control markers, `nil' to disable this feature."
   '(choice (const diff-hl) (const git-gutter) nil))
 
 (spacemacs|defc version-control-diff-side 'right