docs: explain how to watch files inside node_modules

[haoqunjiang]

Description

Closes #5023

Additional context

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

Before submitting the PR, please make sure you do the following

• Read the Contributing Guidelines.
• Read the Pull Request Guidelines and follow the Commit Convention.
• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. fixes #123).
• Ideally, include relevant tests that fail without this PR but pass with it.

[SystemParadox]

NOTE: The documented example does not work. See paulmillr/chokidar#1225.

[patak-dev]

@SystemParadox please create a PR or issue so we can track this, thanks!