man/io_uring_enter.2: document new opcodes
man/io_uring_enter.2: document new opcodes

Add documentation for IORING_OP_SHUTDOWN, IORING_OP_RENAMEAT,
and IORING_OP_UNLINKAT.

Signed-off-by: Jens Axboe <cd8c6775e60d6f67a6984377324e5290df3d5358@kernel.dk>

axboe/liburing,axboe/liburing,axboe/liburing