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/testaudioinfo.c

@@ -30,10 +30,9 @@ print_devices(SDL_bool iscapture)
         int i;
         SDL_Log("Found %d %s device%s:\n", n, typestr, n != 1 ? "s" : "");
         for (i = 0; i < n; i++) {
-            char *name = SDL_GetAudioDeviceName(devices[i]);
+            const char *name = SDL_GetAudioDeviceName(devices[i]);
             if (name) {
                 SDL_Log("  %d: %s\n", i, name);
-                SDL_free(name);
             } else {
                 SDL_Log("  %d Error: %s\n", i, SDL_GetError());
             }