This commit changes the GET REST api so it will accept an optional comma
separated list of enrich policy ids. This change also modifies the
behavior of the GET API in that it will not error if it is passed a bad
enrich id anymore, but will instead just return an empty list.
* Add enrich policy common parameter
* Add enrich APIs to REST APIs index
* Add put enrich policy API docs
* Add get enrich policy API docs
* Add delete enrich policy API docs
* Add execute enrich policy API docs
As mapping types are being removed throughout Elasticsearch, the use of
`_type` in pipeline simulation requests is deprecated. Additionally, the
default `_type` used if one is not supplied has been changed to `_doc` for
consistency with the rest of Elasticsearch.