open.2: Explicitly describe the EBADF error that can occur with openat()

In particular, specifying an invalid file descriptor number
in 'dirfd' can be used as a check that 'pathname' is absolute.

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

man2/open.2

@@ -966,6 +966,19 @@ must be a directory that was opened for reading
 or using the
 .B O_PATH
 flag.
+.PP
+If the pathname given in
+.I pathname
+is relative, and
+.I dirfd
+is not a valid file descriptor, an error
+.RB ( EBADF )
+results.
+(Specifying an invalid file descriptor number in
+.I dirfd
+can be used as a means to ensure that
+.I pathname
+is absolute.)
 .\"
 .SS openat2(2)
 The