add class level comment for RepeatableThread

Summary: Pull Request resolved: https://github.com/facebook/rocksdb/pull/5344

Differential Revision: D15485431

Pulled By: miasantreble

fbshipit-source-id: 9c0f6cf0d826743e743012549976705ceb8cc0c4

util/repeatable_thread.h

@@ -15,6 +15,9 @@
 
 namespace rocksdb {
 
+// Simple wrapper around port::Thread that supports calling a callback every
+// X seconds. If you pass in 0, then it will call your callback repeatedly
+// without delay.
 class RepeatableThread {
  public:
   RepeatableThread(std::function<void()> function,