docs(fast-inbox): mirror the same-block l1ToL2TreeSnapshot semantics in the TS BlockConstantData (A-1432)

Author: spalladino

yarn-project/stdlib/src/rollup/block_constant_data.ts

@@ -13,10 +13,10 @@ export class BlockConstantData {
     /** Archive tree snapshot at the very beginning of the entire rollup. */
     public lastArchive: AppendOnlyTreeSnapshot,
     /**
-     * L1-to-L2 message tree snapshot after this block lands.
-     * For the first block in a checkpoint, this should be the snapshot after inserting the new l1-to-l2 message subtree
-     * into the last l1-to-l2 tree snapshot in `last_archive`.
-     * For subsequent blocks, this should match the snapshot of the previous block.
+     * L1-to-L2 message tree snapshot after this block's own message bundle has been inserted. The AVM validates this
+     * block's l1-to-l2 message read requests against it, so a tx in this block can read the messages this block
+     * inserts (same-block consumption). Every block-root variant that carries txs asserts this equals its computed
+     * post-bundle root, whether or not the block is the first in its checkpoint.
      */
     public l1ToL2TreeSnapshot: AppendOnlyTreeSnapshot,
     /** Root of the verification key tree. */