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_version.h

@@ -143,6 +143,8 @@ extern SDL_DECLSPEC int SDLCALL SDL_GetVersion(void);
  * You shouldn't use this function for anything but logging it for debugging
  * purposes. The string is not intended to be reliable in any way.
  *
+ * The returned string follows the SDL_GetStringRule.
+ *
  * \returns an arbitrary string, uniquely identifying the exact revision of
  *          the SDL library in use.
  *