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/filesystem/winrt/SDL_sysfilesystem.cpp

@@ -93,6 +93,7 @@ static const wchar_t *SDL_WinRTGetFSPathUNICODE(SDL_WinRT_Path pathType)
     return NULL;
 }
 
+// this caches a string until the process ends, so there's no need to use SDL_FreeLater.
 extern "C" const char *SDL_WinRTGetFSPath(SDL_WinRT_Path pathType)
 {
     typedef unordered_map<SDL_WinRT_Path, string> UTF8PathMap;