Docs: introduce TrackerResult.of()

docs/api/tracker-result.md

 
 `result.trackables: Trackable[], read-only`
 
-An array of zero or more [trackables](trackable.md).
+An array of zero or more [trackables](trackable.md).
+
+## Methods
+
+### of
+
+`result.of(trackerType: string): boolean`
+
+Checks if `this` result was generated by tracker of a certain `trackerType`. This is a convenient type guard for TypeScript users. See also: [Tracker.is](./tracker.md#is).
+
+*Since:* 0.4.4
+
+**Returns**
+
+The same as `tracker.is(trackerType)`.
+
+**Example**
+
+```ts
+let result: TrackerResult;
+
+// ...
+
+if(result.of('image-tracker')) {
+    // result is inferred to be an ImageTrackerResult
+    // ...
+}
+else if(result.of('pointer-tracker')) {
+    // result is inferred to be a PointerTrackerResult
+    // ...
+}
+else {
+    // ...
+}
+```

docs/api/tracker.md

 
 `tracker.is(type: string): boolean`
 
-Checks if `this` tracker is of a certain `type`. This works as a convenient type-narrowing method for TypeScript users.
+Checks if `this` tracker is of a certain `type`. This is a convenient type guard for TypeScript users. See also: [TrackerResult.of](./tracker-result.md#of).
 
 *Since:* 0.4.4