Added more comments.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@8637 a5019735-40e9-0310-863c-91ae7b9d1cf9
diff --git a/drd/drd_thread.c b/drd/drd_thread.c
index 82b4d86..b57140f 100644
--- a/drd/drd_thread.c
+++ b/drd/drd_thread.c
@@ -321,6 +321,7 @@
   }
 }
 
+/** Called just before pthread_cancel(). */
 void thread_pre_cancel(const DrdThreadId tid)
 {
   tl_assert(0 <= (int)tid && tid < DRD_N_THREADS