[DOC] clarify LevelDB supported platform

In docs at [1] it states

The LevelDB store is supported only on Linux. For all other JBoss A-MQ-supported platforms, it is provided for technical preview only.

Need to clarify if level DB on a NFSv4 server, the OS of the remote server is not relevant as long as accessed through NFSv4

[1] https://access.redhat.com/documentation/en-US/Red_Hat_JBoss_A-MQ/6.1/html/Configuring_Broker_Persistence/files/LevelDBConfiguration.html