[DOCS] Documented rest.action.multi.allow_explicit_index
[DOCS] Fixed a bad link
Fix test bug: we need at least one parent to have 1 child or more.
Add a benchmark endpoint for elasticsearch that simplifies benchmarking
queries, faceting and suggestions including highlighting etc.

TODOs:
 - add support for pre / post round executions like clear caches or
   expand replicas / wait for green
 - add support to send per query result to another index
 - add support for suggest endpoint to test in isolation
 - add support for stats collection during the benchmark per round?