doc: xdp: Clarify driver implementation for XDP Rx metadata

doc: xdp: Clarify driver implementation for XDP Rx metadata

Clarify that drivers must remove device-reserved metadata from the
data_meta area before passing frames to XDP programs.

Additionally, expand the explanation of how userspace and BPF programs
should coordinate the use of METADATA_SIZE, and add a detailed diagram
to illustrate pointer adjustments and metadata layout.

Also describe the requirements and constraints enforced by
bpf_xdp_adjust_meta().

Signed-off-by: Song Yoong Siang <yoong.siang.song@intel.com>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Link: https://lore.kernel.org/r/20250716154846.3513575-1-yoong.siang.song@intel.com