updates doc on action parameter variable mustache escaping (#88521)

Provides more detail on mustache variable escaping within action parameter templates.

docs/user/alerting/defining-alerts.asciidoc

@@ -59,11 +59,19 @@ Each action type exposes different properties. For example an email action allow
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 image::images/alert-flyout-action-details.png[UI for defining an email action]
 
-Using the https://mustache.github.io/[Mustache] template syntax `{{variable name}}`, you can pass alert values at the time a condition is detected to an action.  Note that using two curly braces will escape any HTML.  Should you need to preserve HTML, use three curly braces (`{{{`). Available variables differ by alert type, and a list can be accessed using the "add variable" button.
+Using the https://mustache.github.io/[Mustache] template syntax `{{variable name}}`, you can pass alert values at the time a condition is detected to an action. Available variables differ by alert type, and the list of available variables can be accessed using the "add variable" button.
 
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 image::images/alert-flyout-action-variables.png[Passing alert values to an action]
 
+Some cases exist where the variable values will be "escaped", when used in a context where escaping is needed:
+
+- For the <<email-action-type, Email>> connector, the `message` action configuration property escapes any characters that would be interpreted as Markdown.
+- For the <<slack-action-type, Slack>> connector, the `message` action configuration property escapes any characters that would be interpreted as Slack Markdown.
+- For the <<webhook-action-type, Webhook>> connector, the `body` action configuration property escapes any characters that are invalid in JSON string values.
+
+Mustache also supports "triple braces" of the form `{{{variable name}}}`, which indicates no escaping should be done at all.  Care should be used when using this form, as it could end up rendering the variable content in such a way as to make the resulting parameter invalid or formatted incorrectly.
+
 You can attach more than one action. Clicking the "Add action" button will prompt you to select another alert type and repeat the above steps again.
 
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