[Soc2014] Official meta API

Since we don't know for sure that it's a `GenericForeignKey` (but often it will be, and we want to provide a useful hint about `GenericRelation` in that case), maybe re-word as follows:

> Field %r does not generate an automatic reverse relation and therefore cannot be used for reverse querying. (If it is a GenericForeignKey, consider adding a GenericRelation.)