Document which functions can block on certain platforms

[daxpedda]

I believe it would be useful for users to know that some functions will block the calling thread until the main thread has executed it's function.

Follow up to #2985.
Cc @madsmtm.

[madsmtm]

I mean there is this:

winit/src/window.rs

Lines 27 to 35 in ee0db52

	/// # Threading
	///
	/// This is `Send + Sync`, meaning that it can be freely used from other
	/// threads.
	///
	/// However, some platforms (macOS, Web and iOS) only allow user interface
	/// interactions on the main thread, so on those platforms, if you use the
	/// window from a thread other than the main, the code is scheduled to run on
	/// the main thread, and your thread may be blocked until that completes.

But do you want to describe exactly which functions will block?

[notgull]

In async-winit, I treat all window functions as blocking. Most of them involving sending a network request which is, by definition, blocking.

[daxpedda]

But do you want to describe exactly which functions will block?

Yes, the idea is to just add it to # Platform-specific.
Unless it is consistent that only functions that return something block, then we could also document just that (this might already be the case, I can double-check later).

I would just like to make sure that users can determine which functions block or not, right now we just say "may".