Document SaveContext.gameMode (#1295)

* Document `SaveContext.gameMode`

* end credits

* Remove no longer needed comment

include/z64save.h

@@ -245,6 +245,13 @@ typedef enum {
     /* 3 */ SUNSSONG_SPECIAL // time does not advance, but signals the song was played. used for freezing redeads
 } SunsSongState;
 
+typedef enum {
+    /* 0 */ GAMEMODE_NORMAL,
+    /* 1 */ GAMEMODE_TITLE_SCREEN,
+    /* 2 */ GAMEMODE_FILE_SELECT, // Note: only instance type transitions swap to file select
+    /* 3 */ GAMEMODE_END_CREDITS
+} GameMode;
+
 typedef enum {
     /* 0 */ LINK_AGE_ADULT,
     /* 1 */ LINK_AGE_CHILD