doc: basic search operator usage

Created by: rvantonder

This is the basic docs for using and/or operators in search. I have other things I want to improve, and will allocate time for that in the next iteration as per #9969.

Layout: syntax references in cs.android and Lucene talk about operators after fields/filters, and I think it makes sense here too.

Update: The setting is active on sourcegraph.com and I tested the examples.