Command buffer and swapchain usage needs clarification in header docs

[thatcosmonaut]

Based on some recent feedback:

So, if I understand correctly, that mean you can only draw to the swapchain texture from the associated command buffer? You should put that in the documentation if it's the case, that will help people understand why they are linked together. I think the first people that will use this API will be the one that use current SDL_renderer but want to improve their graphics with shader. You should not assume any advanced knowledge on how graphic API works.

Not really related to FNA, but to the new SDL_Gpu API, when building a command buffer on a different thread, is it ok to call the submit function on that same thread or does it have to be done on a single thread for all command buffers like Vulkan wants to?

We could clarify these requirements and best practices in the header.