Outdated doc about the parent/child relationship (mapping, indexing, query syntax)

[fdartayre]

The parent-child relationship presented in this section is outdated:
https://www.elastic.co/guide/en/elasticsearch/guide/current/parent-child.html

In particular the mapping described in this page does not work anymore in 6.x:
https://www.elastic.co/guide/en/elasticsearch/guide/current/parent-child-mapping.html

Similarly indexing and query syntax (has child, has parent, etc.) in the parent-child section of the guide should be reviewed as syntax changed.

Updated documentation:
https://www.elastic.co/guide/en/elasticsearch/reference/current/parent-join.html
https://www.elastic.co/guide/en/elasticsearch/reference/current/query-dsl-has-child-query.html
https://www.elastic.co/guide/en/elasticsearch/reference/current/query-dsl-has-parent-query.html
https://www.elastic.co/guide/en/elasticsearch/reference/current/query-dsl-parent-id-query.html