Document how main_thread_id for Windows works (#1951)

src/platform_impl/windows/event_loop.rs

@@ -265,10 +265,36 @@ impl<T> EventLoopWindowTarget<T> {
     }
 }
 
+/// Returns the id of the main thread.
+///
+/// Windows has no real API to check if the current executing thread is the "main thread", unlike
+/// macOS.
+///
+/// Windows will let us look up the current thread's id, but there's no API that lets us check what
+/// the id of the main thread is. We would somehow need to get the main thread's id before a
+/// developer could spin off any other threads inside of the main entrypoint in order to emulate the
+/// capabilities of other platforms.
+///
+/// We can get the id of the main thread by using CRT initialization. CRT initialization can be used
+/// to setup global state within a program. The OS will call a list of function pointers which
+/// assign values to a static variable. To have get a hold of the main thread id, we need to place
+/// our function pointer inside of the `.CRT$XCU` section so it is called before the main
+/// entrypoint.
+///
+/// Full details of CRT initialization can be found here:
+/// https://docs.microsoft.com/en-us/cpp/c-runtime-library/crt-initialization?view=msvc-160
 fn main_thread_id() -> DWORD {
     static mut MAIN_THREAD_ID: DWORD = 0;
+
+    /// Function pointer used in CRT initialization section to set the above static field's value.
+
+    // Mark as used so this is not removable.
     #[used]
     #[allow(non_upper_case_globals)]
+    // Place the function pointer inside of CRT initialization section so it is loaded before
+    // main entrypoint.
+    //
+    // See: https://doc.rust-lang.org/stable/reference/abi.html#the-link_section-attribute
     #[link_section = ".CRT$XCU"]
     static INIT_MAIN_THREAD_ID: unsafe fn() = {
         unsafe fn initer() {