Document contracts.utilities.subscribeToEvent

[wilwade]

Problem

We thought there was a way to be a race condition with starting to get new posts, and getting older posts in subscribeToEvent, but turns out there is not. #179437626

Solution

• Added more documentation to be clear about how ethers works to prevent any overlap.
• Added an extra test around subscribeToEvent
• Updated registry tests to remove test flakiness

with @acruikshank

Double Checks:

• [x] Did you update the changelog?
• [x] Any new modules need to be exported?
• [x] Are new methods in the right module?
• [x] Are new methods/modules in core or not (i.e. porcelain or plumbing)?
• [x] Do you have good documentation on exported methods?

Summary:

• Code documentation and additional test on subscribeToEvent
• Fixed flaky tests around registry

[wilwade]

Changelog isn't updated, and also this is way more than just documentation.

@shannonwells Good call. I finally got the flaky test not failing and forgot to update the PR and changelog. All updated now.