docs: list runtime engine settings that persist in worker mode (#2682)
Worker mode doesn't run PHP's full request shutdown between requests, so
settings changed at runtime (`ini_set()`,
`stream_context_set_default()`, `date_default_timezone_set()`,
`chdir()`, `stream_wrapper_register()`, error and exception handlers)
stay in effect for later requests on the same thread, like static
variables do.
The State persistence section of the worker docs and the isolation note
in the security docs now say so, and warn against putting request- or
user-specific values, such as credentials in the default stream context,
in these settings.