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_hints.h

@@ -3806,6 +3806,8 @@ extern SDL_DECLSPEC void SDLCALL SDL_ResetHints(void);
 /**
  * Get the value of a hint.
  *
+ * The returned string follows the SDL_GetStringRule.
+ *
  * \param name the hint to query
  * \returns the string value of a hint or NULL if the hint isn't set.
  *