car-mirror: Specify the need to send graph roots in each round for push

[justindotpub]

The following text currently appears in the Pull Protocol details, section 3.2.1.

Any available graph roots MUST be sent as an array ([CID]) by the Requestor, regardless of the Requestor's Bloom filter. These form the basis of subquery anchors in the session.

I do not see any similar statement for the Push Protocol in section 3.3.1, and my understanding is that graph roots must always be pushed in each round, regardless of bloom filters, to ensure that returned subgraph roots are not tainted by false positives.

I personally didn't even pick up on the significance of the statement in section 3.2.1 during my reading of the spec, and then I completely missed it when I started by focusing on implementing push and was ignoring the Pull sections of the spec.