docs: clearer comment for expected sequence of altda/ethda blocks

in TestBatcher_FailoverToEthDA_FallbackToAltDA

op-e2e/system/altda/failover_test.go

@@ -51,7 +51,11 @@ func TestBatcher_FailoverToEthDA_FallbackToAltDA(t *testing.T) {
 
 	countEthDACommitment := uint64(0)
 
+	// There is some nondeterministic timing behavior that affects whether the batcher has already
+	// posted batches before seeing the abvoe SetPutFailoverForNRequests behavior change.
 	// Most likely, sequence of blocks will be: altDA, ethDA, ethDA, altDA, altDA, altDA.
+	// 2 ethDA are expected (and checked for) because nChannelsFailover=2, so da-server will return 503 for 2 requests only,
+	// and the batcher always tries altda first for a new channel, and failsover to ethDA only if altda returns 503.
 	for blockNumL1 := startBlockL1.NumberU64(); blockNumL1 < startBlockL1.NumberU64()+6; blockNumL1++ {
 		blockL1, err := geth.WaitForBlock(big.NewInt(0).SetUint64(blockNumL1), l1Client)
 		require.NoError(t, err)