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.)

test/testaudiohotplug.c

@@ -71,7 +71,7 @@ static void iteration(void)
         } else if (e.type == SDL_EVENT_AUDIO_DEVICE_ADDED) {
             const SDL_AudioDeviceID which = (SDL_AudioDeviceID) e.adevice.which;
             const SDL_bool iscapture = e.adevice.iscapture ? SDL_TRUE : SDL_FALSE;
-            char *name = SDL_GetAudioDeviceName(which);
+            const char *name = SDL_GetAudioDeviceName(which);
             if (name) {
                 SDL_Log("New %s audio device at id %u: %s", devtypestr(iscapture), (unsigned int)which, name);
             } else {
@@ -92,7 +92,6 @@ static void iteration(void)
                     /* !!! FIXME: this is leaking the stream for now. We'll wire it up to a dictionary or whatever later. */
                 }
             }
-            SDL_free(name);
         } else if (e.type == SDL_EVENT_AUDIO_DEVICE_REMOVED) {
             dev = (SDL_AudioDeviceID)e.adevice.which;
             SDL_Log("%s device %u removed.\n", devtypestr(e.adevice.iscapture), (unsigned int)dev);