Docs: Updated index-modules/store and setup/configuration

Explain how to set different index storage types, and
added the vm settings required to stop mmapfs from running
out of memory

Closes #6327
More Like This Query: creates only one MLT query per field for all queried items.

Previously, one MLT query per field was created for each item. One issue with
this method is that the maximum number of selected terms was equal to the
number of items times 'max_query_terms'. Instead, users should have direct control
over the maximum number of selected terms allowed, regardless of the number of
queried items.

Another issue related to the previous method is that it could lead to the
selection of rather uninteresting terms, that because they were found in a
particular queried item. Instead, this new procedure enforces the selection of
interesting terms across ALL items, not within each item. This could lead to
search results where the best matching items share commonalities amongst the
best characteristics of all the items.

Closes #6404
Enable DiskThresholdDecider by default, change default limits to 85/90%

Fixes #6200
Fixes #6201
Wait till node is part of cluster state for join process
When a node sends a join request to the master, only send back the response after it has been added to the master cluster state and published.
This will fix the rare cases where today, a join request can return, and the master, since its under load, have not yet added the node to its cluster state, and the node that joined will start a fault detect against the master, failing since its not part of the cluster state.
Since now the join request is longer, also increase the join request timeout default.
closes #6480
Rest-Spec: search was missing the track_scores param