man/io_uring_enter.2: add description of buffer selections
man/io_uring_enter.2: add description of buffer selections

Fixes: https://github.com/axboe/liburing/issues/191
Signed-off-by: Jens Axboe <cd8c6775e60d6f67a6984377324e5290df3d5358@kernel.dk>

axboe/liburing,axboe/liburing,axboe/liburing