Documentation: HID: add a small blurb on udev-hid-bpf

This is the current decision we took: we don't provide automatic loading
of HID-BPF by the kernel directly, but rely on an external tool for it.

This tool is currently udev-hid-bpf, so let's make people aware of it.

Link: https://lore.kernel.org/r/20240608-hid_bpf_struct_ops-v3-11-6ac6ade58329@kernel.org


Signed-off-by: Benjamin Tissoires <bentiss@kernel.org>