[Doc] innobackupex --copy-back option details request

[Muhammad Irfan]

innobackupex --copy-back option works with empty datadir as mentioned in NOTES here http://www.percona.com/doc/percona-xtrabackup/2.2/innobackupex/restoring_a_backup_ibk.html
It would be nice If same NOTE included in option reference here http://www.percona.com/doc/percona-xtrabackup/2.2/innobackupex/innobackupex_option_reference.html#cmdoption-innobackupex--copy-back and in howto document for innobackupex restore here http://www.percona.com/doc/percona-xtrabackup/2.1/howtos/recipes_ibkx_local.html to make documentation more clear on it.

[Alexey Kopytov (akopytov)]

When fixing this bug, also take the --force-non-empty-directories option into account. The option overrides the default behavior described in the note.

[Hrvoje Matijakovic (hrvojem)]

Fixed: https://github.com/percona/percona-xtrabackup/pull/22 and https://github.com/percona/percona-xtrabackup/pull/21

[Shahriyar Rzayev (rzayev-sehriyar)]

Percona now uses JIRA for bug reports so this bug report is migrated to: https://jira.percona.com/browse/PXB-932