diff --git a/MANUAL.txt b/MANUAL.txt
index 7edc6b063..e9519b9b5 100644
--- a/MANUAL.txt
+++ b/MANUAL.txt
@@ -792,6 +792,7 @@ Options affecting specific writers
 
 :   Use ATX-style headers in Markdown and AsciiDoc output. The default is
     to use setext-style headers for levels 1-2, and then ATX headers.
+    (Note: for `gfm` output, ATX headers are always used.)
 
 `--top-level-division=[default|section|chapter|part]`