message: [storage] Fix accumulator docs
  When switching the accumulator index on disk to post-order,
  the documentation comments were never updated.  Fix that and make
  it clear that the in-memory representation differs.
diff: //!
  //! # Position and Physical Representation
  //! As a Merkle Accumulator tree expands to the right and upwards, we number newly frozen nodes
  -//! monotonically. (One way to do it is simply to use in-order index of nodes.) We call the
  -//! stated numbers identifying nodes below simply "Position".
  +//! monotonically. One way to do it is simply to use in-order index of nodes, and this is what
  +//! we do for the in-memory representation. We call the stated numbers identifying nodes below
  +//! simply "Position", and unless otherwise stated, this is the in-order position.
  //!
  -//! And with that we can map a Merkle Accumulator into a key-value storage: key is the position of a
  -//! node, value is hash value it carries.
  +//! For writing to disk however, we write all the children of a node before the parent.
  +//! Thus for disk write order, it is more convenient to use the post-order position as an index.
  +//! And with that we can map a Merkle Accumulator into a key-value storage: key is the post-order
  +//! position of a node, and the value is hash value it carries.
  //!
  //! We store only Frozen nodes, and generate non-Frozen nodes on the fly when accessing the tree.
  //! This way, the physical representation of the tree is append-only, i.e. once written to physical
  //! Here is what we persist for the logical tree in the above example:
  //!
  //! ```text
  -//! Fzn2(3)
  +//! Fzn2(6)
  //! / \
  //! / \
  -//! Fzn1(1) Fzn3(5)
  +//! Fzn1(2) Fzn3(5)
  //! / \ / \
  -//! L0(0) L1(2) L2(4) L3(6) L4(8)
  +//! L0(0) L1(1) L2(3) L3(4) L4(7)
  //! ```
  //!
  //! When the next leaf node is persisted, the physical representation will be:
  //!
  //! ```text
  -//! Fzn2(3)
  +//! Fzn2(6)
  //! / \
  //! / \
  -//! Fzn1(1) Fzn3(5) Fzn4(9)
  +//! Fzn1(2) Fzn3(5) Fzn4(9)
  //! / \ / \ / \
  -//! L0(0) L1(2) L2(4) L3(6) L4(8) L5(10)
  +//! L0(0) L1(1) L2(3) L3(4) L4(7) L5(8)
  //! ```
  //!
  -//! The numbering corresponds to the in-order traversal of the tree.
  +//! The numbering corresponds to the post-order traversal of the tree.
  //!
  //! To think in key-value pairs:
  //! ```text
  //! |<-key->|<--value-->|
  //! | 0 | hash_L0 |
  -//! | 1 | hash_Fzn1 |
  -//! | 2 | hash_L1 |
  +//! | 1 | hash_L1 |
  +//! | 2 | hash_Fzn1 |
  //! | ... | ... |
  //! ```
  