Add new opcodes to io_uring_enter.2 man page

This adds:

IORING_OP_CONNECT
IORING_OP_FALLOCATE
IORING_OP_OPENAT
IORING_OP_CLOSE
IORING_OP_FILES_UPDATE
IORING_OP_STATX
IORING_OP_READ
IORING_OP_WRITE

to the opcode descriptions.

Fixes: https://github.com/axboe/liburing/issues/45
Signed-off-by: Jens Axboe <axboe@kernel.dk>