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/render/SDL_render.c

@@ -780,6 +780,7 @@ int SDL_GetNumRenderDrivers(void)
 #endif
 }
 
+// this returns string literals, so there's no need to use SDL_FreeLater.
 const char *SDL_GetRenderDriver(int index)
 {
 #ifndef SDL_RENDER_DISABLED