[reference][form] Defining exactly what the grouping form option does - closes #320

Author: weaverryan

reference/forms/types/options/grouping.rst.inc

@@ -3,5 +3,4 @@ grouping
 
 **type**: ``integer`` **default**: ``false``
 
-The value set as the ``NumberFormatter::GROUPING_USED`` attribute when using
-the PHP ``NumberFormatter`` class.
+This value is used internally as the ``NumberFormatter::GROUPING_USED`` value when using PHP's ``NumberFormatter`` class. Its documentation is non-existent, but it appears that if you set this to ``true``, numbers will be grouped with a comma or period (depending on your locale): ``12345.123`` would display as ``12,345.123``.