Another round of win32 fixes and documentation.

-) Fixed a few more (much more rare) race conditions on shutdown.
-) Fixed a degenerate case if we create a server but never start it.
diff --git a/src/core/iomgr/pollset_windows.c b/src/core/iomgr/pollset_windows.c
index bea6711..5af0685 100644
--- a/src/core/iomgr/pollset_windows.c
+++ b/src/core/iomgr/pollset_windows.c
@@ -41,6 +41,11 @@
 #include "src/core/iomgr/iomgr_internal.h"
 #include "src/core/iomgr/pollset_windows.h"
 
+/* There isn't really any such thing as a pollset under Windows, due to the
+   nature of the IO completion ports. We're still going to provide a minimal
+   set of features for the sake of the rest of grpc. But grpc_pollset_work
+   won't actually do any polling, and return as quickly as possible. */
+
 void grpc_pollset_init(grpc_pollset *pollset) {
   gpr_mu_init(&pollset->mu);
   gpr_cv_init(&pollset->cv);