media: Documentation: Add note about UVCH length field

The documentation currently describes the UVC length field as the "length
of the rest of the block", which can be misleading. The driver limits the
data copied to a maximum of 12 bytes.

This change adds a clarifying sentence to the documentation to make this
restriction explicit.

Reviewed-by: Hans de Goede <hansg@kernel.org>
Signed-off-by: Ricardo Ribalda <ribalda@chromium.org>
Link: https://lore.kernel.org/r/20250707-uvc-meta-v8-2-ed17f8b1218b@chromium.org


Signed-off-by: Hans de Goede <hansg@kernel.org>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>