Document that ProcessEvents is only needed for AllowProcessEvents callbacks

[kainino0x]

We should explain in docs for wgpuInstanceProcessEvents that it is only needed for AllowProcessEvents callbacks (that aren't waited), or in other words, you don't need it if every future either is waited or has AllowSpontaneous.

(Which led me to another question: is it fine if my app never calls wgpuInstanceProcessEvents or wgpuDeviceTick? If it is not, when should they be called?)

Originally posted by @zcbenz in #491 (comment)

[lokokung]

Agreed! It's important to note that although tehnically we shouldn't need to call ProcessEvents or Tick if all callbacks are AllowSpontaneous, apps that do not continually submit work to the queue (in native) may not see forward progress if they don't call ProcessEvents or Tick at the moment. This will likely be an issue until we resolve a good way to make forward progress in another thread or something.