[DOCS] Document the contracts of the RootMapper API.

Close #7400
More Like This Query: defaults to all possible fields for items

Items with no specified field now defaults to all the possible fields from the
document source. Previously, we had required 'fields' to be specified either
as a top level parameter or for each item. The default behavior is now similar
to the MLT API.

Closes #7382
[ENGINE] Force optimize was not passed to shard request

The force flag to trigger optimiz calls of a single segment for upgrading
etc. was never passed on to the shard request.

Closes #7404