Clarify requirements to claim a window

[flibitijibibo]

Let's make promises that claiming a window that isn't in a specific state will just fail outright. Like, the window must be created with (or without?) specific flags, it must not have a 2D renderer associated with it, and it might get recreated if necessary (although I assume recreation is less likely in the post-OpenGL era).

This is probably where we can say "Android is weird and needs SDL_WINDOW_VULKAN" for example