[doc] Configuring file-based cache stores - should mention which store is used

The documentation  [1] does not have a hint that <file-store> will use soft-index.

As well `shared="false"` might lead to confusion, it is the default anyway, but neither the file-store nor the single-file-store are allowed to configure it as shared - it will throw an expcetion!

 [1]https://infinispan.org/docs/stable/titles/configuring/configuring.html#file-stores_persistence

Writer's note: Target guide for update = https://infinispan.org/docs/stable/titles/configuring/configuring.html