Improve docs: tell customers about likely CORS issues

[Alex-At-Home]

errors like:

ERROR [10:[email protected]]: status = [unknown], msg = [No Living connections], query = [/_cat/indices?format=json]

probably mean one of two things - either you're pointing at the Kibana, or the cluster does not have CORS setup (which seems to be necessary from 7.0 up)

[Alex-At-Home]

Note I believe that the following CORS settings are desired for max security:

http.cors.enabled: true
http.cors.allow-origin: "https://n-zps6avz5zp6tamkkazpk2btgl7vaa4bckyf5ngi-*-script.googleusercontent.com"
http.cors.max-age: 86400
http.cors.allow-methods: GET
http.cors.allow-headers: "Authorization, Access-Control-Allow-Headers, Origin, Accept, X-Requested-With, Content-Type, Content-Length, Access-Control-Request-Method, Access-Control-Request-Headers"
http.cors.allow-credentials: true

(Testeed with a different user, and we had the same first bit but i had 0hl vs his 3hl in the middle?!