Introduce formal policy for APIs that return strings.

This declares that any `const char *` returned from SDL is owned by SDL, and
promises to be valid _at least_ until the next time the event queue runs, or
SDL_Quit() is called, even if the thing that owns the string gets destroyed
or changed before then.

This is noted in the headers as "the SDL_GetStringRule", so this will both be
greppable to find a detailed explaination in docs/README-strings.md and
wikiheaders will automatically turn it into a link we can point at the
appropriate documentation.

Fixes #9902.

(and several FIXMEs, both known and yet-undocumented.)

src/haptic/SDL_haptic.c

@@ -100,7 +100,7 @@ const char *SDL_GetHapticInstanceName(SDL_HapticID instance_id)
     if (SDL_GetHapticIndex(instance_id, &device_index)) {
         name = SDL_SYS_HapticName(device_index);
     }
-    return name;
+    return name ? SDL_FreeLater(SDL_strdup(name)) : NULL;
 }
 
 SDL_Haptic *SDL_OpenHaptic(SDL_HapticID instance_id)
@@ -338,7 +338,7 @@ void SDL_CloseHaptic(SDL_Haptic *haptic)
     }
 
     /* Free the data associated with this device */
-    SDL_free(haptic->name);
+    SDL_FreeLater(haptic->name);  // this pointer is handed to the app in SDL_GetHapticName()
     SDL_free(haptic);
 }