Document the possible EPERM return.

Submitted by:	imp@village.org (Warner Losh)

lib/libc/sys/chroot.2

@@ -71,6 +71,8 @@ will fail and the root directory will be unchanged if:
 .Bl -tag -width [ENOTDIR]
 .It Bq Er ENOTDIR
 A component of the path name is not a directory.
+.It Bq Er EPERM
+The effective user ID is not the super-user.
 .It Bq Er EINVAL
 The pathname contains a character with the high-order bit set.
 .It Bq Er ENAMETOOLONG