Adding documentation for global action configuration options (#83557)

* Adding documentation for global action configuration options

* Update docs/user/alerting/defining-alerts.asciidoc

Co-authored-by: gchaps <33642766+gchaps@users.noreply.github.com>

* incorporating PR feedback

Co-authored-by: gchaps <33642766+gchaps@users.noreply.github.com>

Author: bmcconaghy

docs/user/alerting/defining-alerts.asciidoc

@@ -74,6 +74,21 @@ image::images/alert-flyout-add-action.png[You can add multiple actions on an ale
 Actions are not required on alerts. In some cases you may want to run an alert without actions first to understand its behavior, and configure actions later.
 ==============================================
 
+[float]
+=== Global actions configuration
+Some actions configuration options apply to all actions.
+If you are using an *on-prem* Elastic Stack deployment, you can set these in the kibana.yml file.
+If you are using a cloud deployment, you can set these via the console.
+
+Here's a list of the available global configuration options and an explanation of what each one does:
+
+* `xpack.actions.allowedHosts`: Specifies an array of host names which actions such as email, Slack, PagerDuty, and webhook can connect to. An element of * indicates any host can be connected to. An empty array indicates no hosts can be connected to. Default: [ {asterisk} ]
+* `xpack.actions.enabledActionTypes`: Specifies to an array of action types that are enabled. An {asterisk} indicates all action types registered are enabled. The action types that {kib} provides are: .server-log, .slack, .email, .index, .pagerduty, .webhook. Default: [ {asterisk} ]
+* `xpack.actions.proxyUrl`: Specifies the proxy URL to use, if using a proxy for actions.
+* `xpack.actions.proxyHeader`: Specifies HTTP headers for proxy, if using a proxy for actions.
+* `xpack.actions.proxyRejectUnauthorizedCertificates`: Set to `false` to bypass certificate validation for proxy, if using a proxy for actions.
+* `xpack.actions.rejectUnauthorized`: Set to `false` to bypass certificate validation for actions.
+
 [float]
 === Managing alerts