Improve the comment of util/thread_local.h

Summary: Improve the comment of util/thread_local.h

Test Plan: n/a

Reviewers: ljin

Reviewed By: ljin

Differential Revision: https://reviews.facebook.net/D25449

util/thread_local.h

@@ -26,13 +26,14 @@ namespace rocksdb {
 // (2) a ThreadLocalPtr is destroyed
 typedef void (*UnrefHandler)(void* ptr);
 
-// Thread local storage that only stores value of pointer type. The storage
+// ThreadLocalPtr stores only values of pointer type.  Different from
-// distinguish data coming from different thread and different ThreadLocalPtr
+// the usual thread-local-storage, ThreadLocalPtr has the ability to
-// instances. For example, if a regular thread_local variable A is declared
+// distinguish data coming from different threads and different
-// in DBImpl, two DBImpl objects would share the same A. ThreadLocalPtr avoids
+// ThreadLocalPtr instances.  For example, if a regular thread_local
-// the confliction. The total storage size equals to # of threads * # of
+// variable A is declared in DBImpl, two DBImpl objects would share
-// ThreadLocalPtr instances. It is not efficient in terms of space, but it
+// the same A.  However, a ThreadLocalPtr that is defined under the
-// should serve most of our use cases well and keep code simple.
+// scope of DBImpl can avoid such confliction.  As a result, its memory
+// usage would be O(# of threads * # of ThreadLocalPtr instances).
 class ThreadLocalPtr {
  public:
   explicit ThreadLocalPtr(UnrefHandler handler = nullptr);