fix(node): Add comments explaining value of the jest boilerplate

[a-roberts]

calls out the use of baselines too

Contributes to: /issues/1949

• Other (Provide information)

I was initially confused by the Jest boilerplate's intent - spoke with Tom and I think this explains it well and helped me to understand it so I can show our teams how to make use of it and the accessibility checker

This PR is related to the following issue(s):

Fixes #1949

Testing reference:

I've checked this looks fine in VSCode's markdown editor as well as running the tests with the expected behaviour

I have conducted the following for this PR:

• I validated this code in Chrome and FF
• I validated this fix in my local env
• I provided details for testing
• This PR has been reviewed and is ready for test
• I understand that the title of this PR will be used for the next release notes.