ZEND_ASYNC_IO_PRESERVE_FD carried no prose, and its real contract turned out to be narrower than every caller assumed: it reaches only a descriptor the reactor closes on its own account. A descriptor adopted by a libuv stream handle is outside it — the handle owns the descriptor and uv_close closes it whatever the flag says.

That gap cost a real defect in true-async/server (issue php#293): a log sink passed the flag for a TCP socket, kept the PHP stream on the strength of it, and the socket was closed twice — WSAENOTSOCK on Windows, a silent EBADF on POSIX, which is the worse half because the descriptor number is reused and the late close lands on another connection.

Header comment only, no behaviour change.