vague words on record_type_id

as discussed with Eileen [here](http://forum.civicrm.org/index.php/topic,31282.0.html)

xml/schema/Activity/ActivityContact.xml

@@ -54,7 +54,7 @@
     <name>record_type_id</name>
     <type>int unsigned</type>
     <title>Record Type ID</title>
-    <comment>The record type id for this row</comment>
+    <comment>Contents vary depending on the activity type. Code using custom activities may use this field as they choose fit.</comment>
     <pseudoconstant>
       <optionGroupName>activity_contacts</optionGroupName>
     </pseudoconstant>