Update zero-initialization example

doc/note/zero-initialization.md

diff --git a/doc/note/zero-initialization.md b/doc/note/zero-initialization.md
index be7d6a0..fcf28f4 100644
--- a/doc/note/zero-initialization.md
+++ b/doc/note/zero-initialization.md
@@ -10,8 +10,9 @@
 uint32_t flags = 0;
 etc
 
-// Uncomment this line to take the option.
-// flags |= WUFFS_INITIALIZE__LEAVE_INTERNAL_BUFFERS_UNINITIALIZED;
+// Setting this flag bit takes the option. Its presence or absence has no
+// effect if the WUFFS_INITIALIZE__ALREADY_ZEROED flag bit is also set.
+flags |= WUFFS_INITIALIZE__LEAVE_INTERNAL_BUFFERS_UNINITIALIZED;
 
 wuffs_foo__bar__initialize(etc, flags);
 ```