diff --git a/Doc/lib/libos.tex b/Doc/lib/libos.tex
index 50dad08..04acce6 100644
--- a/Doc/lib/libos.tex
+++ b/Doc/lib/libos.tex
@@ -875,7 +875,8 @@
 but makes all intermediate-level directories needed to contain the
 leaf directory.  Throws an \exception{error} exception if the leaf
 directory already exists or cannot be created.  The default \var{mode}
-is \code{0777} (octal).
+is \code{0777} (octal).  On some systems, \var{mode} is ignored.
+Where it is used, the current umask value is first masked out.
 \versionadded{1.5.2}
 \versionchanged[This function now handles UNC paths correctly]{2.3}
 \end{funcdesc}