KahaDB configuration forceRecoverIndex is not documented

This came out of ENTMQ-688.

In both the enterprise, and community docs the option forceRecoverIndex for KahaDB is not discussed.

http://activemq.apache.org/persistence.html

https://access.redhat.com/site/documentation/en-US/Red_Hat_JBoss_A-MQ/6.1/html/Configuring_Broker_Persistence/files/KahaDBConfiguration.html

I know this feature works in A-MQ 6.0, but I'm not sure how early it was implemented. If we could at least get it added to A-MQ 6, and 6.1 docs, that would be great.