Documentation: hid: intel-ish-hid: Document PRODUCT_FAMILY firmware matching

Documentation: hid: intel-ish-hid: Document PRODUCT_FAMILY firmware matching

Document the ISH firmware filename matching rules, including the
new PRODUCT_FAMILY-based patterns and their search order.

This aligns the documentation with the driver behavior and provides
clear guidance for vendors supplying custom ISH firmware.

Signed-off-by: Vishnu Sankar <vishnuocv@gmail.com>
Acked-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Signed-off-by: Jiri Kosina <jkosina@suse.com>