Add Doc Comments for Raw Block Data Storage

[sergerad]

... add doc comments (maybe at the module level), explaining how we save raw block data. I believe currently it is something like this:

• Raw block data is saved under [store_dir]/[epoch]/block_[block_num].dat file.
• For proven blocks, the proof data is saved under [store_dir]/epoch[epoch_num]/proof_[block_num].dat file.

Where epoch_num is the 16 most significant bits of the block number, and both epoch_num and block_num are formatted as hex strings.

Originally posted by @bobbinth in #1725 (comment)

Also this comment: #1725 (comment)