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/testmultiaudio.c

@@ -57,7 +57,7 @@ test_multi_audio(SDL_AudioDeviceID *devices, int devcount)
 #endif
 
     for (i = 0; i < devcount; i++) {
-        char *devname = SDL_GetAudioDeviceName(devices[i]);
+        const char *devname = SDL_GetAudioDeviceName(devices[i]);
 
         SDL_Log("Playing on device #%d of %d: id=%u, name='%s'...", i, devcount, (unsigned int) devices[i], devname);
 
@@ -82,7 +82,6 @@ test_multi_audio(SDL_AudioDeviceID *devices, int devcount)
             SDL_Log("done.");
             SDL_DestroyAudioStream(stream);
         }
-        SDL_free(devname);
         stream = NULL;
     }