runtime: document that LockOSThread in init locks to thread

This is more or less implied by the spec language on initialization,
but restate it for clarity.

Fixes #23112

Change-Id: Ibe5385acafe4eac38823de98a025cd37f7a77d3b
Reviewed-on: https://go-review.googlesource.com/103399Reviewed-by: Austin Clements <austin@google.com>

src/runtime/proc.go

@@ -3472,6 +3472,10 @@ func dolockOSThread() {
 // If the calling goroutine exits without unlocking the thread,
 // the thread will be terminated.
 //
+// All init functions are run on the startup thread. Calling LockOSThread
+// from an init function will cause the main function to be invoked on
+// that thread.
+//
 // A goroutine should call LockOSThread before calling OS services or
 // non-Go library functions that depend on per-thread state.
 func LockOSThread() {