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/testautomation_audio.c

@@ -365,7 +365,7 @@ static int audio_enumerateAndNameAudioDevices(void *arg)
 {
     int t;
     int i, n;
-    char *name;
+    const char *name;
     SDL_AudioDeviceID *devices = NULL;
 
     /* Iterate over types: t=0 output device, t=1 input/capture device */
@@ -385,7 +385,6 @@ static int audio_enumerateAndNameAudioDevices(void *arg)
                 SDLTest_AssertCheck(name != NULL, "Verify result from SDL_GetAudioDeviceName(%i) is not NULL", i);
                 if (name != NULL) {
                     SDLTest_AssertCheck(name[0] != '\0', "verify result from SDL_GetAudioDeviceName(%i) is not empty, got: '%s'", i, name);
-                    SDL_free(name);
                 }
             }
         }