doc: fix information about including coverage files

PR-URL: #54527
Reviewed-By: Moshe Atlow <moshe@atlow.co.il>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

doc/api/test.md

@@ -431,7 +431,8 @@ command-line flag, code coverage is collected and statistics are reported once
 all tests have completed. If the [`NODE_V8_COVERAGE`][] environment variable is
 used to specify a code coverage directory, the generated V8 coverage files are
 written to that directory. Node.js core modules and files within
-`node_modules/` directories are not included in the coverage report. If
+`node_modules/` directories are, by default, not included in the coverage report.
+However, they can be explicity included via the [`--test-coverage-include`][] flag. If
 coverage is enabled, the coverage report is sent to any [test reporters][] via
 the `'test:coverage'` event.
 
@@ -3268,6 +3269,7 @@ Can be used to abort test subtasks when the test has been aborted.
 [`--experimental-test-coverage`]: cli.md#--experimental-test-coverage
 [`--import`]: cli.md#--importmodule
 [`--test-concurrency`]: cli.md#--test-concurrency
+[`--test-coverage-include`]: cli.md#--test-coverage-include
 [`--test-name-pattern`]: cli.md#--test-name-pattern
 [`--test-only`]: cli.md#--test-only
 [`--test-reporter-destination`]: cli.md#--test-reporter-destination