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_keyboard.h

@@ -109,6 +109,8 @@ extern SDL_DECLSPEC SDL_KeyboardID *SDLCALL SDL_GetKeyboards(int *count);
  *
  * This function returns "" if the keyboard doesn't have a name.
  *
+ * The returned string follows the SDL_GetStringRule.
+ *
  * \param instance_id the keyboard instance ID
  * \returns the name of the selected keyboard, or NULL on failure; call
  *          SDL_GetError() for more information.
@@ -255,6 +257,8 @@ extern SDL_DECLSPEC SDL_Scancode SDLCALL SDL_GetScancodeFromKey(SDL_Keycode key)
  *
  * See SDL_Scancode for details.
  *
+ * The returned string follows the SDL_GetStringRule.
+ *
  * **Warning**: The returned name is by design not stable across platforms,
  * e.g. the name for `SDL_SCANCODE_LGUI` is "Left GUI" under Linux but "Left
  * Windows" under Microsoft Windows, and some scancodes like
@@ -295,6 +299,8 @@ extern SDL_DECLSPEC SDL_Scancode SDLCALL SDL_GetScancodeFromName(const char *nam
  *
  * See SDL_Scancode and SDL_Keycode for details.
  *
+ * The returned string follows the SDL_GetStringRule.
+ *
  * \param key the desired SDL_Keycode to query
  * \returns a pointer to a UTF-8 string that stays valid at least until the
  *          next call to this function. If you need it around any longer, you