Document support for waitFor (ampproject#8718)

extensions/amp-analytics/amp-analytics.md

@@ -328,12 +328,13 @@ The element visibility trigger can be configured for any AMP element or a docume
 
 Notice that selector can be used to only specify a single element, not a collection. The element can be either an [AMP extended  element](https://github.xom/ampproject/amphtml/blob/master/spec/amp-tag-addendum.md#amp-specific-tags) or a document root.
 
-The element visibility trigger waits for element's [`ini-load`](#initial-load-trigger) signal before matching the `visibilitySpec`.
+The element visibility trigger waits for the signal specified by the `waitFor` property in `visibilitySpec` before tracking element visibility. If `waitFor` is not specified, it waits for element's [`ini-load`](#initial-load-trigger) signal. See `waitFor` docs for more details.
 
 
 <strong><a id="visibility-spec"></a>Visibility Spec</strong>
 
 The `visibilitySpec` is a set of conditions and properties that can be applied to `visible` or `hidden` triggers to change when they fire. If multiple properties are specified, they must all be true in order for a request to fire. Configuration properties supported in `visibilitySpec` are:
+  - `waitFor` This property indicates that the visibility trigger should wait for a certain signal before tracking visibility. The supported values are     `none`, `ini-load` and `render-start`. If `waitFor` is undefined, it is defaulted to [`ini-load`](#initial-load-trigger) when selector is specified, or to `none` otherwise.
   - `continuousTimeMin` and `continuousTimeMax` These properties indicate that a request should be fired when (any part of) an element has been within the viewport for a continuous amount of time that is between the minimum and maximum specified times. The times are expressed in milliseconds. The `continuousTimeMin` is defaulted to 0 when not specified.
   - `totalTimeMin` and `totalTimeMax` These properties indicate that a request should be fired when (any part of) an element has been within the viewport for a total amount of time that is between the minimum and maximum specified times. The times are expressed in milliseconds. The `totalTimeMin` is defaulted to 0 when not specified.
   - `visiblePercentageMin` and `visiblePercentageMax` These properties indicate that a request should be fired when the proportion of an element that is visible within the viewport is between the minimum and maximum specified percentages. Percentage values between 0 and 100 are valid. Note that the lower bound (`visiblePercentageMin`) is inclusive while the upper bound (`visiblePercentageMax`) is not. When these properties are defined along with other timing related properties, only the time when these properties are met are counted. They default to 0 and 100 when not specified.
@@ -347,6 +348,7 @@ In addition to the conditions above, `visibilitySpec` also enables certain varia
     "request": "pageview",
     "selector": "#ad1",
     "visibilitySpec": {
+      "waitFor": "ini-load",
       "visiblePercentageMin": 20,
       "totalTimeMin": 500,
       "continuousTimeMin": 200