error docs

docs/content/error/ngOptions/iexp.ngdoc

@@ -1,4 +1,13 @@
 @ngdoc error
 @name ngOptions:iexp
 @fullName Invalid Expression
-@description
+@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' under {@link api/ng.directive:select ng.directive:select}.