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_platform.h

@@ -48,12 +48,14 @@ extern "C" {
  * - "iOS"
  * - "Android"
  *
+ * The returned string follows the SDL_GetStringRule.
+ *
  * \returns the name of the platform. If the correct platform name is not
  *          available, returns a string beginning with the text "Unknown".
  *
  * \since This function is available since SDL 3.0.0.
  */
-extern SDL_DECLSPEC const char * SDLCALL SDL_GetPlatform (void);
+extern SDL_DECLSPEC const char * SDLCALL SDL_GetPlatform(void);
 
 /* Ends C function definitions when using C++ */
 #ifdef __cplusplus