[DOCS] added `discovery.zen.publish_timeout` to the dynamic settings list
[DOCS] Documented `discovery.zen.publish_timeout` setting
Improved ClusterSettingsTests

more assertAcked, checked that the `discovery.zen.publish_timeout` has been changed in `DiscoverySettings`, removed restriction on number of nodes, doesn't seem needed.
Minor cleanups in ShardsLimitAllocationDecider
[DOCS] added coming tags for `zen.discovery.publish_timeout` made dynamic
Made highlight query also work in the percolate api.