Improve documentation/error handling around use of `host` and `url`

[robbavey]

When using this plugin, either host or url is required for the plugin to be able to connect to an appsearch instance.

The documentation for the individual configuration states that host is used with Elastic Appsearch managed service, and that url is used with self-managed Elastic Appsearch, but this is not surfaced earlier in the documentation, either explicitly or in an example configuration, which would likely prove helpful here.

Additionally, the error message surfaced to the user when they supply a configuration containing neither a host nor url setting is not helpful, providing no actionable information for the user:

[2020-10-19T17:42:17,378][ERROR][logstash.agent           ] Failed to execute action {:id=>:main, :action_type=>LogStash::ConvergeResult::FailedAction, :message=>"Could not execute action: PipelineAction::Create<main>, action_result: false", :backtrace=>nil}

[ppf2]

I previously created a documentation PR related to this, would be great to get a review: https://github.com/logstash-plugins/logstash-output-elastic_app_search/pull/19 thx!

[robbavey]

cc @karenzone - the PR above created/linked by @ppf2 has some more additional information that will be helpful to users too