doc: correct _FORTIFY_SOURCE doc in features.h

diff --git a/include/features.h b/include/features.h
index 7d2c7a4..07c73b3 100644 (file)
--- a/include/features.h
+++ b/include/features.h
@@ -57,7 +57,8 @@
                        __STRICT_ANSI__).
 
    _FORTIFY_SOURCE     Add security hardening to many library functions.
-                       Set to 1 or 2; 2 performs stricter checks than 1.
+                       Set to 1, 2 or 3; 3 performs stricter checks than 2, which
+                       performs stricter checks than 1.
 
    _REENTRANT, _THREAD_SAFE
                        Obsolete; equivalent to _POSIX_C_SOURCE=199506L.