Update oak-jackrabbit documentation for revisionGC in RDBMK

Documentation at [0] and [1] represents Continuous Revision Garbage Collection is valid only for mongoMK not for rdbMK. But both implementation share common code for this, so this is valid for both(see [2]). Hence documentation has to be updated accordingly. 

[0] : https://jackrabbit.apache.org/oak/docs/nodestore/documentmk.html#Revision_Garbage_Collection

[1] : https://jackrabbit.apache.org/oak/docs/osgi_config.html#document-node-store

[2] : https://github.com/apache/jackrabbit-oak/blob/trunk/oak-store-document/src/main/java/org/apache/jackrabbit/oak/plugins/document/DocumentNodeStoreService.java#L812