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.)

include/SDL3/SDL_render.h

@@ -166,8 +166,7 @@ extern SDL_DECLSPEC int SDLCALL SDL_GetNumRenderDrivers(void);
  * "direct3d12" or "metal". These never have Unicode characters, and are not
  * meant to be proper names.
  *
- * The returned value points to a static, read-only string; do not modify or
- * free it!
+ * The returned string follows the SDL_GetStringRule.
  *
  * \param index the index of the rendering driver; the value ranges from 0 to
  *              SDL_GetNumRenderDrivers() - 1