Document rocksdb_wal_recovery_mode

Our KB:

https://mariadb.com/kb/en/myrocks-system-variables/#rocksdb_wal_recovery_mode

is not much helpful while describing this setting (and the enterprise documentation on it is not any better).

Please extend it with description that can be found here:

https://github.com/facebook/mysql-5.6/wiki/MyRocks-Crash-Recovery

and

https://github.com/facebook/rocksdb/wiki/WAL-Recovery-Modes

There is also missing correlation between these two, which goes as follows:

rocksdb_wal_recovery_mode = 1 (default)
wal_recovery_mode=kAbsoluteConsistency

rocksdb_wal_recovery_mode = 0
wal_recovery_mode=kTolerateCorruptedTailRecords

rocksdb_wal_recovery_mode = 2
wal_recovery_mode=kPointInTimeRecovery

rocksdb_wal_recovery_mode = 3
wal_recovery_mode=kSkipAnyCorruptedRecords

It can be verified by checking the $datadir/.rocksdb/OPTIONS-* file for wal_recovery_mode= value after using particular rocksdb_wal_recovery_mode setting.