[DOC]Create documentation for _msearch API

[kolchfa-aws]

What do you want to do?

• [ ] Request a change to existing documentation
• [x ] Add new documentation
• [ ] Report a technical problem with the documentation
• [ ] Other

Tell us about your request. Provide a summary of the request and all versions that are affected.

POST /_msearch
GET /<index>/_msearch
POST /<index>/_msearch
GET /_msearch/template
POST /_msearch/template
GET /<index>/_msearch/template
POST /<index>/_msearch/template