add note about not invalidating valid and available blocks

specs/deneb/fork-choice.md

@@ -47,6 +47,8 @@ Initially, verification requires every verifying actor to retrieve all matching
 
 The block MUST NOT be considered valid until all valid `Blob`s have been downloaded. Blocks that have been previously validated as available SHOULD be considered available even if the associated `Blob`s have subsequently been pruned.
 
+*Note*: Extraneous or invalid Blobs received on the p2p network MUST NOT invalidate a block that is otherwise valid and available.
+
 ```python
 def is_data_available(beacon_block_root: Root, blob_kzg_commitments: Sequence[KZGCommitment]) -> bool:
     # `retrieve_blobs_and_proofs` is implementation and context dependent