[storage] Fix accumulator docs
[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.

libra/libra,aptos-labs/aptos-core,libra/libra,aptos-labs/aptos-core,libra/libra,aptos-labs/aptos-core,aptos-labs/aptos-core,aptos-labs/aptos-core,libra/libra,libra/libra,aptos-labs/aptos-core,aptos-labs/aptos-core