Add some docs to prevent a time loop. (#4702)

* Add some docs to prevent a time loop.

* Review remarks.

polkadot/node/network/availability-recovery/src/lib.rs

@@ -569,8 +569,20 @@ const fn is_unavailable(
 }
 
 /// Re-encode the data into erasure chunks in order to verify
-/// the root hash of the provided merkle tree, which is built
+/// the root hash of the provided Merkle tree, which is built
 /// on-top of the encoded chunks.
+///
+/// This (expensive) check is necessary, as otherwise we can't be sure that some chunks won't have
+/// been tampered with by the backers, which would result in some validators considering the data
+/// valid and some invalid as having fetched different set of chunks. The checking of the Merkle
+/// proof for individual chunks only gives us guarantees, that we have fetched a chunk belonging to
+/// a set the backers have committed to.
+///
+/// NOTE: It is fine to do this check with already decoded data, because if the decoding failed for
+/// some validators, we can be sure that chunks have been tampered with (by the backers) or the
+/// data was invalid to begin with. In the former case, validators fetching valid chunks will see
+/// invalid data as well, because the root won't match. In the latter case the situation is the
+/// same for anyone anyways.
 fn reconstructed_data_matches_root(
 	n_validators: usize,
 	expected_root: &Hash,