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/audio/SDL_audio.c

@@ -131,6 +131,7 @@ int SDL_GetNumAudioDrivers(void)
     return num_drivers;
 }
 
+// this returns string literals, so there's no need to use SDL_FreeLater.
 const char *SDL_GetAudioDriver(int index)
 {
     if (index >= 0 && index < SDL_GetNumAudioDrivers()) {
@@ -139,6 +140,7 @@ const char *SDL_GetAudioDriver(int index)
     return NULL;
 }
 
+// this returns string literals, so there's no need to use SDL_FreeLater.
 const char *SDL_GetCurrentAudioDriver(void)
 {
     return current_audio.name;
@@ -521,7 +523,7 @@ static void DestroyPhysicalAudioDevice(SDL_AudioDevice *device)
     SDL_DestroyMutex(device->lock);
     SDL_DestroyCondition(device->close_cond);
     SDL_free(device->work_buffer);
-    SDL_free(device->name);
+    SDL_FreeLater(device->name);  // this pointer is handed to the app during SDL_GetAudioDeviceName
     SDL_free(device);
 }
 
@@ -1402,12 +1404,12 @@ SDL_AudioDevice *SDL_FindPhysicalAudioDeviceByHandle(void *handle)
     return SDL_FindPhysicalAudioDeviceByCallback(TestDeviceHandleCallback, handle);
 }
 
-char *SDL_GetAudioDeviceName(SDL_AudioDeviceID devid)
+const char *SDL_GetAudioDeviceName(SDL_AudioDeviceID devid)
 {
-    char *retval = NULL;
+    const char *retval = NULL;
     SDL_AudioDevice *device = ObtainPhysicalAudioDevice(devid);
     if (device) {
-        retval = SDL_strdup(device->name);
+        retval = device->name;
     }
     ReleaseAudioDevice(device);