Document elf_message scripting system (#1398)

* `cUpElfMsgs` -> `cUpMsgs`

Motivation:
ElfMessage is the name of the system that handles c-up messages from navi, but also messages from child/adult saria
so it would make sense to name this after the system name like "ElfMsg"
but "cUpElf" primarily reads like (to me) "c up navi" since the actor implementing navi is EnElf

I think it'd be better to clear that confusion between "navi" and "elfmsg system" by just not using "elf" in the name

* Standardize c-up navi messages ElfMessage data to be named after "CUp"

* -> "Navi"

* Make symbols in `elf_message_` segments static

* new docs

* change commands, format

* introduce navi file id

* use enum

* remove concat

* add clarifying comment for skip

* concat cmd type

* Hints -> QuestHint

* rename command header

* s -> g

* missed enum name

* singular

* missed one

Co-authored-by: Dragorn421 <Dragorn421@users.noreply.github.com>

src/code/z_scene.c

@@ -1,7 +1,7 @@
 #include "global.h"
 #include "vt.h"
 
-RomFile sNaviMsgFiles[];
+RomFile sNaviQuestHintFiles[];
 
 s32 Object_Spawn(ObjectContext* objectCtx, s16 objectId) {
     u32 size;
@@ -231,8 +231,8 @@ void Scene_CommandSpecialFiles(PlayState* play, SceneCmd* cmd) {
         gSegments[5] = VIRTUAL_TO_PHYSICAL(play->objectCtx.status[play->objectCtx.subKeepIndex].segment);
     }
 
-    if (cmd->specialFiles.cUpElfMsgNum != 0) {
-        play->cUpElfMsgs = Play_LoadFile(play, &sNaviMsgFiles[cmd->specialFiles.cUpElfMsgNum - 1]);
+    if (cmd->specialFiles.naviQuestHintFileId != NAVI_QUEST_HINTS_NONE) {
+        play->naviQuestHints = Play_LoadFile(play, &sNaviQuestHintFiles[cmd->specialFiles.naviQuestHintFileId - 1]);
     }
 }
 
@@ -498,7 +498,7 @@ void (*gSceneCmdHandlers[SCENE_CMD_ID_MAX])(PlayState*, SceneCmd*) = {
     Scene_CommandMiscSettings,        // SCENE_CMD_ID_MISC_SETTINGS
 };
 
-RomFile sNaviMsgFiles[] = {
+RomFile sNaviQuestHintFiles[] = {
     ROM_FILE(elf_message_field),
     ROM_FILE(elf_message_ydan),
     ROM_FILE_UNSET,