[DOCS] Need elaboration in 'secure reporting endpoint' section to include both JKS or PEM options are supported

[elasticmachine]

Original comment by @hoigau:

In the latest documentation:

https://www.elastic.co/guide/en/kibana/5.5/securing-reporting.html

it's written like the truststore is the only option, but in fact it's also fine if the PEM equivalent settings are setup to trust the kibana server chain. It'd be good to have it elaborated that both the truststore or PEM option are supported, customer just need to setup one or another.

Configure Watcher to trust the Kibana server’s certificate by adding it to the Watcher truststore on each node:

Import the Kibana server certificate into the Watcher truststore using Java Keytool:

    keytool -importcert -keystore watcher-truststore.jks -file server.crt

   Note.  If the truststore doesn’t already exist, it is created.
    
Make sure the xpack.http.ssl.truststore.path setting in elasticsearch.yml specifies the location of the Watcher truststore. 

[elasticmachine]

Pinging @elastic/kibana-docs (Team:Docs)

[jrodewig]

This touches Kibana docs. Transferring to the Kibana repo from the Elasticsearch repo.