docs(minerr): add description for $rootScope:infdig

docs/content/error/rootScope/infdig.ngdoc

@@ -2,3 +2,13 @@
 @name $rootScope:infdig
 @fullName Infinite $digest Loop
 @description
+
+This error occurs when the application's model becomes unstable and each `$digest` cycle triggers a state change and subsequent `$digest` cycle. Angular detects this situation and prevents an infinite loop from causing the browser to become unresponsive.
+
+For example, the situation can occur by setting up a watch on a path and subsequently updating the same path when the value changes.
+
+```
+$scope.$watch('foo', function() {
+  $scope.foo = $scope.foo + 1;
+});
+```