docs(tlv): update piece content metadata length

[gaius-qi]

Description

This pull request updates the documentation for the PieceContent value format in the src/tlv/piece_content.rs file to accurately reflect the actual sizes of several fields. The main changes clarify the byte sizes for metadata, digest, and parent ID length fields, as well as correct the cost field size.

Documentation updates for value format:

• Changed the "Metadata Length," "Digest Length," and "Parent ID Length" fields from 8 bytes to 4 bytes in the documentation to match the actual implementation.
• Corrected the "Digest" field description to indicate it is variable length (was previously fixed at 32 bytes).
• Updated the "Cost" field size from 64 bytes to 8 bytes in the documentation.

Related Issue

Motivation and Context

Screenshots (if appropriate)

[chlins]

lgtm

[chlins]

lgtm