[6.2] Adds `wait_for_no_initializing_shards` to cluster health API

[codebrain]

https://www.elastic.co/guide/en/elasticsearch/reference/current/release-notes-6.2.0.html

Cluster: Adds wait_for_no_initializing_shards to cluster health API - elastic/elasticsearch#27489

[codebrain]

Should be picked up by API Generator when re-running over 6.2.0 json specification.

@russcam - shall I close this?

[codebrain]

Closing as changes picked up by API generator re-run on 6.2 specifications.