Improve --no-lock description in docs

Reporting a part of https://bugs.launchpad.net/percona-xtrabackup/+bug/792407/comments/10 as a separate bug:

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http://www.percona.com/doc/percona-xtrabackup/innobackupex/innobackupex_option_reference.html#cmdoption-innobackupex--no-lock

I think here you should add a sentence that hints to using --safe-slave-backup:

"If you are considering to use --no-lock because your backups are failing to acquire the lock, this could be because of incoming replication events preventing the lock from succeeding. Please try using --safe-slave-backup to momentarily stop the replication slave thread, this may help the backup to succeed and you then don't need to resort to using --no-lock.
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