Document some Boost data (#593)

* Run formatter

* Document some boost info

include/object_properties.h

@@ -172,7 +172,16 @@ typedef struct ObjPropertyTimeTrial {
 
 typedef struct ObjPropertyBoost {
     struct Object *obj;
-    s32 unk4;
+    union {
+        s32 indexes;  // Keep this for compatibility
+        // This is a bitfield definition for the bits in indexes.
+        // It won't match when used, but it's useful for understanding the structure.
+        struct {
+            s32 racerIndex : 4;   // bits 28-31  ((indexes >> 28) & 0xF)
+            s32 vertIndex  : 14;  // bits 14-27  ((indexes >> 14) & 0x3FFF)
+            s32 trisIndex  : 14;  // bits 0-13   (indexes & 0x3FFF)
+        };
+    };
 } ObjPropertyBoost;
 
 typedef struct ObjProperties {