XDebug default doesn't work

The docs say that `--xdebug` will turn xdebug on, but in my experience this isn't the case. I need to pass `--xdebug=debug` (which is supposed to be the default) or the debug server doesn't work (i.e. PHPStorm never receives notification of an incoming XDebug thing).

Start by locating the documentation for the --xdebug option and the entry point that handles this flag. Compare the documented default with the behavior when --xdebug is used, then verify that the debug server sends a notification to PHPStorm without requiring --xdebug=debug.

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