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_mouse.h

@@ -152,6 +152,8 @@ extern SDL_DECLSPEC SDL_MouseID *SDLCALL SDL_GetMice(int *count);
  *
  * This function returns "" if the mouse doesn't have a name.
  *
+ * The returned string follows the SDL_GetStringRule.
+ *
  * \param instance_id the mouse instance ID
  * \returns the name of the selected mouse, or NULL on failure; call
  *          SDL_GetError() for more information.