Add doc strings into task_runner.h

BUG: 24267885
Change-Id: I4f42194296abb180f1543b4a243aa8f064515450
Reviewed-on: https://weave-review.googlesource.com/1155
Reviewed-by: Alex Vakulenko <avakulenko@google.com>

libweave/include/weave/task_runner.h

diff --git a/libweave/include/weave/task_runner.h b/libweave/include/weave/task_runner.h
index 845d9f7..43b81b7 100644
--- a/libweave/include/weave/task_runner.h
+++ b/libweave/include/weave/task_runner.h
@@ -15,8 +15,13 @@
 
 namespace weave {
 
+// Interface with methods to post tasks into platform-specific message loop of
+// the current thread.
 class TaskRunner {
  public:
+  // Posts tasks to be executed with the given delay.
+  // |from_here| argument is used for debugging and usually just provided by
+  // FROM_HERE macro. Implementation may ignore this argument.
   virtual void PostDelayedTask(const tracked_objects::Location& from_here,
                                const base::Closure& task,
                                base::TimeDelta delay) = 0;