compiler, runtime: move defer notes to the runtime file

This seems like the more appropriate place to describe the
implementation of defer.

src/runtime/defer.go

@@ -1,5 +1,17 @@
 package runtime
 
+// Defer statements are implemented by transforming the function in the
+// following way:
+//   * Creating an alloca in the entry block that contains a pointer (initially
+//     null) to the linked list of defer frames.
+//   * Every time a defer statement is executed, a new defer frame is created
+//     using alloca with a pointer to the previous defer frame, and the head
+//     pointer in the entry block is replaced with a pointer to this defer
+//     frame.
+//   * On return, runtime.rundefers is called which calls all deferred functions
+//     from the head of the linked list until it has gone through all defer
+//     frames.
+
 import "unsafe"
 
 type deferContext unsafe.Pointer