diff --git a/doc/lispref/os.texi b/doc/lispref/os.texi
index 3e16ac0..2546074 100644
--- a/doc/lispref/os.texi
+++ b/doc/lispref/os.texi
@@ -3146,6 +3146,10 @@ respective library like @file{inotify}, @file{kqueue},
 @file{gfilenotify}, or @file{w32notify} statically.  These libraries
 enable watching of filesystems on the local machine.
 
+The variable @var{file-notify--library} is non-nil when Emacs has been
+compiled with file notification support, and value is the name of the
+low-level file notification package to be used for local file systems.
+
 It is also possible to watch filesystems on remote machines,
 @pxref{Remote Files,, Remote Files, emacs, The GNU Emacs Manual}.
 This does not depend on one of the libraries linked to Emacs.