docs(columnDef): update name regarding uniqueness

Add a warning about the name needing to be unique to
all instances of grids on a page because it can be used
as a key for retrieving custom sort algorithms, etc..

src/js/core/factories/GridColumn.js

@@ -18,17 +18,23 @@ angular.module('ui.grid')
    * @ngdoc property
    * @name name
    * @propertyOf ui.grid.class:GridColumn
-   * @description (mandatory) each column should have a name, although for backward
-   * compatibility with 2.x name can be omitted if field is present
+   * @description (mandatory) Each column should have a name, although for backward
+   * compatibility with 2.x name can be omitted if field is present.
+   *
+   * Important - This must be unique to each column on a web page since it can
+   * be used as a key for retrieving information such as custom sort algorithms.
    *
    */
 
   /**
    * @ngdoc property
    * @name name
    * @propertyOf ui.grid.class:GridOptions.columnDef
-   * @description (mandatory) each column should have a name, although for backward
-   * compatibility with 2.x name can be omitted if field is present
+   * @description (mandatory) Each column should have a name, although for backward
+   * compatibility with 2.x name can be omitted if field is present.
+   *
+   * Important - This must be unique to each column on a web page since it can
+   * be used as a key for retrieving information such as custom sort algorithms.
    *
    */