io_uring/zcrx: document area chunking parameter

struct io_uring_zcrx_ifq_reg::rx_buf_len is used as a hint specifying
the kernel what buffer size it should use. Document the API and
limitations.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>

Author: isilence

Documentation/networking/iou-zcrx.rst

@@ -196,6 +196,26 @@ Return buffers back to the kernel to be used again::
   rqe->len = cqe->res;
   IO_URING_WRITE_ONCE(*refill_ring.ktail, ++refill_ring.rq_tail);
 
+Area chunking
+-------------
+
+zcrx splits the memory area into fixed-length physically contiguous chunks.
+This limits the maximum buffer size returned in a single io_uring CQE. Users
+can provide a hint to the kernel to use larger chunks by setting the
+``rx_buf_len`` field of ``struct io_uring_zcrx_ifq_reg`` to the desired length
+during registration. If this field is set to zero, the kernel defaults to
+the system page size.
+
+To use larger sizes, the memory area must be backed by physically contiguous
+ranges whose sizes are multiples of ``rx_buf_len``. It also requires kernel
+and hardware support. If registration fails, users are generally expected to
+fall back to defaults by setting ``rx_buf_len`` to zero.
+
+Larger chunks don't give any additional guarantees about buffer sizes returned
+in CQEs, and they can vary depending on many factors like traffic pattern,
+hardware offload, etc. It doesn't require any application changes beyond zcrx
+registration.
+
 Testing
 =======