docs: mention new viewport trigger syntax in docs (angular#64130)

Updates the docs to mention the new options for the `viewport` trigger.

PR Close angular#64130

Author: crisbeto

adev/src/content/guide/templates/defer.md

@@ -177,6 +177,24 @@ Alternatively, you can specify a [template reference variable](/guide/templates/
 }
 ```
 
+If you want to customize the options of the `IntersectionObserver`, the `viewport` trigger supports passing in an object literal. The literal supports all properties from the second parameter of `IntersectionObserver`, except for `root`. When using the object literal notation, you have to pass your trigger using the `trigger` property.
+
+```angular-html
+<div #greeting>Hello!</div>
+
+<!-- With options and a trigger -->
+@defer (on viewport({trigger: greeting, rootMargin: '100px', threshold: 0.5})) {
+  <greetings-cmp />
+}
+
+<!-- With options and an implied trigger -->
+@defer (on viewport({rootMargin: '100px', threshold: 0.5})) {
+  <greetings-cmp />
+} @placeholder {
+  <div>Implied trigger</div>
+}
+```
+
 #### `interaction`
 
 The `interaction` trigger loads the deferred content when the user interacts with the specified element through `click` or `keydown` events.