Docs: wgpuDeviceCreateBuffer returns null if mapAtCreation fails

[kainino0x]

Issue: #57

[kainino0x]

We should also specify either that there's an out-of-memory error scope error, or implementation-defined logging, even though it's the only thing that can cause createBuffer to return null (at least for now). Adding needs docs to do this.

[kainino0x]

I don't think it should be an out-of-memory error because the JS spec doesn't produce one in this case. It just throws a RangeError from createBuffer(). https://gpuweb.github.io/gpuweb/#dom-gpudevice-createbuffer