Document src/n64dd/n64dd_error_headers.c (#2667)

* Document src/n64dd/n64dd_error_headers.c

* Incorporate feedback from PR review.

(see also https://github.com/zeldaret/oot/pull/2667)

* Put translation of gN64DDLocalizedErrorNumberHeaders[0] in a line comment

(From feedback to https://github.com/zeldaret/oot/pull/2667)

include/n64dd.h

@@ -167,7 +167,7 @@ extern s32 B_801E0F64;
 extern void (*D_801D2EB4)(void*, void*, void*);
 
 // Error messages
-extern const char* D_801D2ED0[];        // "Error Number    " array
+extern const char* gN64DDLocalizedErrorNumberHeaders[]; // "Error Number    " array
 extern const char* D_801D2EE0[2][8][4]; // Array of error message strings
 
 // Error textures

src/n64dd/n64dd_801C9B70.c

@@ -61,7 +61,7 @@ void func_801C9C74(u8* dest, u8 value, u32 count) {
 
 // n64ddError_GetErrorHeader
 const char* func_801C9CA4(void) {
-    return D_801D2ED0[func_801C9C48()];
+    return gN64DDLocalizedErrorNumberHeaders[func_801C9C48()];
 }
 
 // n64ddError_WriteNumberJP

src/n64dd/n64dd_error_headers.c

@@ -1,4 +1,13 @@
 #include "n64dd.h"
 
-// Padding in .rodata suggests that these are in a separate file
-const char* D_801D2ED0[4] = { "エラー番号　　　", "Error Number    ", NULL, NULL };
+// Padding in .rodata suggests that this array is in a separate file
+
+/**
+ * Region-specific "error number" headers used in N64DD error handling.
+ * The first one is used if the current region is Japan (gCurrentRegion == REGION_JP),
+ * the second otherwise.
+ */
+const char* gN64DDLocalizedErrorNumberHeaders[2] = {
+    "エラー番号　　　", // "Error Number"
+    "Error Number    ",
+};