open.2: Clarify that openat()'s dirfd must be opened with O_RDONLY or O_PATH

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man2/open.2

@@ -959,6 +959,13 @@ referred to by the file descriptor
 the calling process, as is done by
 .BR open ()
 for a relative pathname).
+In this case,
+.I dirfd
+must be a directory that was opened for reading
+.RB ( O_RDONLY )
+or using the
+.B O_PATH
+flag.
 .\"
 .SS openat2(2)
 The