Add context.alerts to rule creation UI and API docs (#448)

Author: marshallmain

docs/detections/api/rules/rules-api-create.asciidoc

@@ -453,6 +453,7 @@ All text fields (such as `message` fields) can contain placeholders for rule
 and alert details:
 
 * `{{state.signals_count}}`: Number of alerts detected
+* `{{context.alerts}}`: Array of detected alerts
 * `{{{context.results_link}}}`: URL to the alerts in {kib}
 * `{{context.rule.anomaly_threshold}}`: Anomaly threshold score above which
 alerts are generated ({ml} rules only)

docs/detections/rules-ui-create.asciidoc

@@ -436,6 +436,7 @@ minutes at 14:03 but it does not run until 14:04, it will run again at 14:09.
 These placeholders can be added to <<rule-notifications, rule action>> fields:
 
 * `{{state.signals_count}}`: Number of alerts detected
+* `{{context.alerts}}`: Array of detected alerts
 * `{{{context.results_link}}}`: URL to the alerts in {kib}
 * `{{context.rule.anomaly_threshold}}`: Anomaly threshold score above which
 alerts are generated ({ml} rules only)