Refer to get_preferred_pillage() in enum extra_cause documenting comment
get_preferred_pillage() defines extra cause preference order for pillaging. Tell that in fc_types.h comment near where extra_cause is defined so that someone adding new cause types will know to check it.
Not targeting to branches older than S3_1 as those are already d3f, and no cause types are expected to be added to them.
get_preferred_pillage() defines extra cause preference order for pillaging. Tell that in fc_types.h comment near where extra_cause is defined so that someone adding new cause types will know to check it.