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.)

src/camera/SDL_camera.c

@@ -63,6 +63,7 @@ int SDL_GetNumCameraDrivers(void)
     return SDL_arraysize(bootstrap) - 1;
 }
 
+// this returns string literals, so there's no need to use SDL_FreeLater.
 const char *SDL_GetCameraDriver(int index)
 {
     if (index >= 0 && index < SDL_GetNumCameraDrivers()) {
@@ -71,6 +72,7 @@ const char *SDL_GetCameraDriver(int index)
     return NULL;
 }
 
+// this returns string literals, so there's no need to use SDL_FreeLater.
 const char *SDL_GetCurrentCameraDriver(void)
 {
     return camera_driver.name;