doc: exempt test/doc only changes from 48-hr rule

If a change affects only specific parts of the tests and or
documentation, and in particular not of the runtime code itself,
it should often be okay to land it early.

The primary goal of the 48-hour rule is to ensure that people
who are invested in certain parts of Node have a reasonable chance
to weigh in based on their usage of Node; if the API is not
touched in a change, that is arguably much less of an issue.

In particular, this:

- Should help avoid excessive nitpicking.
- Helps newcomers, since their first PRs often touch only those areas
  and a direct success is very motivating.
- Helps with the amount of pull requests created by events such a
  Code & Learn.

PR-URL: https://github.com/nodejs/node/pull/16135


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