SDL3: Document that the `override` parameter of `SDL_OpenTitleStorage()` requires a path separator on the end

[nightmareci]

I sometimes have to override the title storage root in my project, e.g. supporting Linux package manager setups having title storage installed into /usr/share/game_name, and ran into some confusing errors in the process, concluding that I didn't include a path separator on the end of the override path. I think that requirement should be documented in SDL's documentation, to help people in the future not have to deal with the issue.

Other functions taking directory paths might have the same requirement, in SDL_storage.h, or in other headers; where required, document the requirement, of course.