fix some style issues with comments

Summary:
D397 added some extra blank lines between comments and the block of code they apply to.

This commit fixes this. A single blank line is used after a comment when it applies to multiple object/classes following it. When the comment describes a single function, it is transformed into a docstring.

I only touched code that already diverged from Core. There are a couple of other comments that could be transformed into docstrings.

Test Plan: arc lint

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D8274