Create document API _id not as per documentation

[geekthanos]

Describe the feature: _id generated is not a UUID as expected per the document

Elasticsearch version (bin/elasticsearch --version): 6.2.1

Plugins installed: []

JVM version (java -version): 1.8

OS version (uname -a if on a Unix-like system): OSX

Description of the problem including expected versus actual behavior:
As per the documentation, https://www.elastic.co/guide/en/elasticsearch/reference/current/docs-index_.html, auto generation id should give a UUID like 6a8ca01c-7896-48e9-81cc-9f70661fcb32. However, it gives YnwksmIBSI4iEKmi9Xrs which looks like a GUID.

[jasontedor]

I opened #29461.