doc: note that watch mode reruns all files with isolation none

[ethanresnick]

The Watch mode section promises that a change "will rerun the tests affected by
the change", with no isolation caveat, and the isolation option is documented
purely as where tests run. But the two interact: watchFiles' 'changed'
handler restarts only the files owning the change under isolation: 'process',
and a single child running the whole file list under isolation: 'none'
(runner.js#L659-L672).

Verified on v26.5.0 with two test files, editing only the first:

node --test --watch a.test.js b.test.js
  ✔ A-ran        ℹ tests 1

node --test --test-isolation=none --watch a.test.js b.test.js
  ✔ A-ran  ✔ B-ran        ℹ tests 2

Docs-only; this describes current behavior rather than changing it. Whether the
behavior itself should change is #65037 — if it does, this paragraph goes away
with it.

[nodejs-github-bot]

Review requested:

• @nodejs/test_runner

[Copilot]

Pull request overview

Updates the node:test documentation to clarify how watch mode interacts with the --test-isolation setting, aligning the docs with the current behavior when running tests in a shared process.

Changes:

• Add a note to the Watch mode section explaining that rerun scope differs between --test-isolation=process (affected files) and --test-isolation=none (all files).

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