Document the recovery process with relay-log-info-repository=TABLE

**Reported in Launchpad by Miguel Angel Nieto last update 01-08-2017 07:00:29

When relay-log-info-repository=TABLE is enabled, the mysql instance recovered from the backup shows errors in the error log:

2016-08-09 12:39:03 69297 [ERROR] Failed to open the relay log '/data/mysql-relay-bin.004389' (relay_log_pos 56365993).
2016-08-09 12:39:03 69297 [ERROR] Could not find target log file mentioned in relay log info in the index file '/data/mysql-relay-bin.index' during relay log initialization.
2016-08-09 12:39:03 69297 [ERROR] Failed to initialize the master info structure

And there are two workarounds:

1- Connect and run reset slave all.

or

2- Have relay_log_recovery enabled so MySQL takes care if it automatically.

The error happens because when you take a backup, the relay log information is also backed up, but new empty relay log that starts from number 000001 is created, so there is a mismatch. This should be documented somehow, so users searching for that error and xtrabackup can find what is happening, why and how to solve it.