[chronik-client] Apply standardized-ish commenting

Summary:
Use single line comments // for comments of 1 or 2 lines. Use multi-line commments /* */ for longer comments, not including comments on the first and last line of multi-line block

This is the kind of thing that should be linted but I was not able to find support for this in either eslint or prettier. Because this issue has not come up in any other repo, just patching it here and moving on is best available approach.

Test Plan: Confirm changes are only to comments

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14884