wifi: iwlwifi: fw: add clarifying comments about iwl_fwrt_dump_data

wifi: iwlwifi: fw: add clarifying comments about iwl_fwrt_dump_data

iwl_fw_dbg_collect_sync() unconditionally accesses 'trig' at
the beginning of the function, even though the data has an
inner union and 'trig' isn't necessarily always used, it can
be 'desc' instead. Add comments clarifying why this is OK,
that 'trig'/'desc' must be first in the struct and how in it
the use of the union's sub-structs is differentiated.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Miri Korenblit <miriam.rachel.korenblit@intel.com>
Link: https://msgid.link/20240319100755.49c3a39737cc.I91c588e0d66b49f0ee9103e8d4a0e501c2fd36d6@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>