docs(error): added description for ngOptions/iexp.ngdoc

Closes #3431

docs/content/error/ngOptions/iexp.ngdoc

@@ -2,3 +2,12 @@
 @name ngOptions:iexp
 @fullName Invalid Expression
 @description
+This error occurs when 'ngOptions' is passed an expression that isn't in an expected form.
+
+Here's an example of correct syntax:
+
+```
+<select ng-model="color" ng-options="c.name for c in colors">
+```
+
+For more information on valid expression syntax, see 'ngOptions' in {@link api/ng.directive:select select} directive docs.