docs: clarify checksum usage
//! The contents of a single Block are as follows:
//!
//! - Checksum (4 bytes BE): can be used to verify integrity of the rest of the block.
+//! Note: the checksum does not include itself in its calculation.
//! - Block ID (4 bytes BE): the ID of the series associated with the block.
//! - Min timestamp (8 bytes BE): timestamp of the earliest value in the block.
//! - Max timestamp (8 bytes BE): timestamp of the latest value in the block.
