Mediawiki documentation says:
Extension registration ignores any key in extension.json starting with '@' in the top-level structure or under config, so you can put comments in those parts of the JSON file.
However, if you actually try to put a @note into extension.json, you get this:
1) ExtensionJsonValidationTest::testPassesValidation with data set #21 ('/workspace/src/extensions/Wik...n.json') Failed asserting that '/workspace/src/extensions/WikibaseCirrusSearch/extension.json did not pass validation.  The property @note is not defined and the definition does not allow additional properties ' matches expected false. /workspace/src/tests/phpunit/structure/ExtensionJsonValidationTest.php:66 /workspace/src/maintenance/doMaintenance.php:94
and the CI build fails. This should be supported by the validator.