[DOC] Document combined_fields query

[harrisonengel]

What do you want to do?

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

Tell us about your request.
I'm submitting a PR to add the combined_fields query. This is the ticket to add Documentation for it.

Version: latest

What other resources are available?

[kolchfa-aws]

@harrisonengel Would you be able to put up a PR for this documentation?