Update code search to Universal Code Search in some places

Created by: christinaforney

At the request of @tammy-zhu we would like to update reference of code search to Universal Code Search in some places.

Here is the search that shows all the places "code search" appears in our docs: https://sourcegraph.com/search?q=context:global+repo:github.com/sourcegraph/sourcegraph%24%405afac98+file:doc/.*+%22code+search%22&patternType=regexp

Please review and let me know if there are any other places that need to be updated.