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/core/android/SDL_android.c

@@ -2417,6 +2417,7 @@ void SDL_AndroidBackButton(void)
     (*env)->CallStaticVoidMethod(env, mActivityClass, midManualBackButton);
 }
 
+// this caches a string until the process ends, so there's no need to use SDL_FreeLater.
 const char *SDL_AndroidGetInternalStoragePath(void)
 {
     static char *s_AndroidInternalFilesPath = NULL;
@@ -2516,6 +2517,7 @@ int SDL_AndroidGetExternalStorageState(Uint32 *state)
     return 0;
 }
 
+// this caches a string until the process ends, so there's no need to use SDL_FreeLater.
 const char *SDL_AndroidGetExternalStoragePath(void)
 {
     static char *s_AndroidExternalFilesPath = NULL;