chore: align lean specification with e9bb80

[syjn99]

What was wrong?

Fixes #744, and also matches with current lean specification (commit hash: e9bb80cb5fef3b6a1a2aae009a37cf86f002e431).

How was it fixed?

• Did some double-check with current lean spec.
• Changed history_length (= mcache_len) to 6.
• Added docstrings with the specification link.

Note

I'm quite sure the networking part would need some changes:

1. Use fork digest instead of devnet{N}: Agreed with @KolbyML's comment.
2. Add topic for calculating message-id: Making it consistent with altair spec.

But I didn't include it in this PR.

To-Do

• I have read CONTRIBUTING.md.
• This PR title follows conventional commits.

[KolbyML]

looks good