stat.2: document SFBSD_MNTROOT

Reviewed by:	sobomax, markj, olce
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Differential revision:	https://reviews.freebsd.org/D59910

lib/libsys/stat.2

@@ -365,6 +365,10 @@ field contains the following flags, defined in
 .Bl -tag -width indent
 .It Dv SFBSD_NAMEDATTR
 The file is a named attribute or a named attribute directory.
+.It Dv SFBSD_MNTPOINT
+The file is a mount point.
+The reported file attributes describe the root node of the file system
+mounted on it.
 .El
 .Sh RETURN VALUES
 .Rv -std