--no-lock description should reference xtrabackup_binlog_pos_innodb

The description of --no-lock in the manual does not say when and how users can get binlog coordinates when taking a backup with --no-lock.

We should make it clear that xtrabackup_binlog_info is not created when --no-lock is used (because SHOW MASTER STATUS may be inconsistent), but under certain conditions xtrabackup_binlog_pos_innodb can be used instead to get consistent binlog coordinates as described at http://www.percona.com/doc/percona-xtrabackup/2.1/xtrabackup_bin/working_with_binary_logs.html