document dynamic reloading for trusted certs

[elasticmachine]

Original comment by @jpcarey:

For the truststore / JKS, this file is dynamically reloaded during runtime for 5.0+

For xpack.ssl.certificate_authorities, this setting is not a dynamic setting, however the individual files will get reloaded if modified.

If there are any other keystore / truststore features that can dynamically reload content, we should list that detail on the https://www.elastic.co/guide/en/elasticsearch/reference/master/security-settings.html pages.

cc: @jaymode

[jpcarey]

It would be awesome if we could get this documented.

[debadair]

[doc-issue-triage]