Document sentence triggers (#27929)

* Document sentence triggers

* Clean up sentence trigger docs

source/_docs/automation/trigger.markdown

@@ -24,6 +24,7 @@ An automation can be triggered by an event, a certain entity state, at a given t
 - [Geolocation trigger](#geolocation-trigger)
 - [Device triggers](#device-triggers)
 - [Calendar trigger](#calendar-trigger)
+- [Sentence trigger](#sentence-trigger)
 - [Multiple triggers](#multiple-triggers)
 - [Multiple Entity IDs for the same Trigger](#multiple-entity-ids-for-the-same-trigger)
 
@@ -892,6 +893,30 @@ automation:
 See the [Calendar](/integrations/calendar/) integration for more details on event triggers and the
 additional event data available for use by an automation.
 
+## Sentence trigger
+
+A sentence trigger fires when [Assist](/voice_control/) matches a sentence from a voice assistant using the default [conversation agent](/integrations/conversation/). 
+
+Sentences are allowed to use some basic [template syntax](https://developers.home-assistant.io/docs/voice/intent-recognition/template-sentence-syntax/#sentence-templates-syntax) like optional and alternative words. For example, `[it's ]party time` will match both "party time" and "it's party time".
+
+```yaml
+automation:
+  trigger:
+    - platform: conversation
+      command:
+        - "[it's ]party time"
+        - "happy (new year|birthday)"
+```
+
+The sentences matched by this trigger will be:
+
+- party time
+- it's party time
+- happy new year
+- happy birthday
+
+Punctuation and casing are ignored, so "It's PARTY TIME!!!" will also match.
+
 ## Multiple triggers
 
 It is possible to specify multiple triggers for the same rule. To do so just prefix the first line of each trigger with a dash (-) and indent the next lines accordingly. Whenever one of the triggers fires, [processing](#what-are-triggers) of your automation rule begins.