get_all_records_where documentation

[BobBall]

There is currently no external documentation for get_all_records_where but several usage examples:

https://github.com/openvswitch/ovs/blob/master/xenserver/usr_share_openvswitch_scripts_ovs-xapi-sync#L86
https://git.openstack.org/cgit/openstack/nova/tree/nova/virt/xenapi/vm_utils.py#n754

Given the performance benefits of using this method in some situations I think this should be externally documented, even with the current caveats around using the DB fields (e.g. name__label) rather than externally visible fields.