Documentation pass centered on z_player_lib (#1230)

include/z64save.h

@@ -101,7 +101,7 @@ typedef struct {
 
 typedef struct {
     /* 0x0000 */ s32 entranceIndex; // start of `save` substruct, originally called "memory"
-    /* 0x0004 */ s32 linkAge;
+    /* 0x0004 */ s32 linkAge; // 0: Adult; 1: Child (see enum `LinkAge`)
     /* 0x0008 */ s32 cutsceneIndex;
     /* 0x000C */ u16 dayTime; // "zelda_time"
     /* 0x0010 */ s32 nightFlag;